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Caffeine Consumption through Coffee: Content in the Beverage, Metabolism, Health Benefits and Risks

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Caffeine (1,3,7-trimethylxanthine) is the most consumed psychoactive substance in the world, acting by means of antagonism to adenosine receptors, mainly A1 and A2A. Coffee is the main natural source of the alkaloid which is quite soluble and well extracted during the brew’s preparation. After consumption, caffeine is almost completely absorbed and extensively metabolized in the liver by phase I (cytochrome P450) enzymes, mainly CYP1A2, which appears to be polymorphically distributed in human populations. Paraxanthine is the major caffeine metabolite in plasma, while methylated xanthines and methyluric acids are the main metabolites excreted in urine. In addition to stimulating the central nervous system, caffeine exerts positive effects in the body, often in association with other substances, contributing to prevention of several chronic diseases. The potential adverse effects of caffeine have also been extensively studied in animal species and in humans. These aspects will be approached in the present review.
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Beverages 2019, 5, 37; doi:10.3390/beverages5020037 www.mdpi.com/journal/beverages
Review
Caffeine Consumption through Coffee: Content in
the Beverage, Metabolism, Health Benefits and Risks
Juliana dePaula and Adriana Farah *
Laboratório de Química e Bioatividade de Alimentos e Núcleo de Pesquisa em Café (NUPECAFÉ), Instituto de
Nutrição, Universidade Federal do Rio de Janeiro, Avenida Carlos Chagas Filho, 373, CCS, Bl.J, Rio de Janeiro
21941-902, Brazil; julianadepaula.nutricao@gmail.com
* Correspondence: afarah@nutricao.ufrj.br; Tel.: +55-21-30396449
Received: 14 February 2019; Accepted: 11 April 2019; Published: 1 June 2019
Abstract: Caffeine (1,3,7-trimethylxanthine) is the most consumed psychoactive substance in the
world, acting by means of antagonism to adenosine receptors, mainly A1 and A2A. Coffee is the
main natural source of the alkaloid which is quite soluble and well extracted during the brew’s
preparation. After consumption, caffeine is almost completely absorbed and extensively
metabolized in the liver by phase I (cytochrome P450) enzymes, mainly CYP1A2, which appears to
be polymorphically distributed in human populations. Paraxanthine is the major caffeine
metabolite in plasma, while methylated xanthines and methyluric acids are the main metabolites
excreted in urine. In addition to stimulating the central nervous system, caffeine exerts positive
effects in the body, often in association with other substances, contributing to prevention of
several chronic diseases. The potential adverse effects of caffeine have also been extensively
studied in animal species and in humans. These aspects will be approached in the present review.
Keywords: caffeine; coffee; consumption; metabolism; health benefits; potential adverse effects
1. Introduction
Caffeine consumption is an ancient habit. Different cultures discovered that chewing seeds,
barks, or leaves of certain plants containing this substance had the effects of easing fatigue,
increasing awareness, and elevating mood [1]. Caffeine (1,3,7-trimethylxanthine) is a heterocyclic
organic compound with a purine base called xanthine, consisted of a pyrimidine ring linked to an
imidazole ring [2] (Figure 1). Caffeine is known as an alkaloid because it is a secondary plant
metabolite derived from purine nucleotides, with a heterocyclic nitrogen atom (definition of true
alkaloid) [2,3]. However, because it does not have the incorporation of an amino acid in its
biosynthesis [2], some authors call it a pseudo-alkaloid [4].
Figure 1. Caffeine chemical structure.
Caffeine is the most abundant methylxanthine in foods [5]. It is present in nearly 100 species in
13 orders of the plant kingdom [6]. Although Coffea species are the major sources, it is also
abundantly found in Camelia sinensis, maté (Ilex paraguariensis), coca (Erythroxylon coca) and Coffea
leaves, in cocoa (Theobroma cacao) and guaraná (Pauliniacupana) seeds, and in kola (Cola sp.) nuts [2,7],
Beverages 2019, 5, 37 2 of 51
in addition to other less significant sources. It is also found in several commercial non-alcoholic
beverages, powders, capsules and in association with therapeutic drugs [5].
Although there are reports on the use of coffee at least since the 9th century, caffeine was only
understood as a substance and component of food matrices in the middle of the 19th century. The
compound was first isolated by the German researcher Ferdinand Runge (17951867), under the
request of the chemistry and technology professor Johann Wolfgang Döbereiner (17801849) and the
philosopher Wolfgang von Goethe (17491832) [8]. Caffeine was first called kaffein, which later
became caffeine in English and was included in the medical vocabulary in 1823 [9]. In the
following years (1827 to 1865), the same compound was isolated from other plants with different
names at times [10].
The chemical structure of caffeine was first proposed in 1875 by Ludwig Medicus (18471915),
who, unusual for that time, deduced it from the already known pure compound [10]. The structures
of caffeine and other methylxanthines were validated in 1882 by Hermann Emil Fischer (18521919),
who published a series of studies on purine synthesis that were cited in his Nobel Prize in Chemistry
in 1902. [11,12]. Since then, studies on caffeine have evolved largely, together with its broad
consumption worldwide. Today, much is known about the compositional, metabolic and
physiological aspects. This review will summarize these aspects, with a focus on contents in
different beverages, health benefits and potential adverse effects associated with caffeine
consumption through coffee.
2. Chemical Aspects and Analysis of Caffeine
Caffeine is colorless at room temperature, odorless, and bitter [13]. It dissolves well in boiling
water, and its solubility is increased by the addition of acids and formation of complexes, such as
benzoate, citrate, and salicylate, at high temperatures (1%, w/v, at 15 °C and 10% at 60 °C) [14]. Better
solubility is achieved in chloroform at room temperature [2]. In aqueous solution, at physiological
pH, caffeine is a non-ionized compound. The melting point is 234 to 239 °C and the temperature of
sublimation, at atmospheric pressure, is about 178 to 180 °C [2]. Reports on caffeine ultraviolet (UV)
absorption region differ slightly in the literature. A reasonable wavelength interval between 250280
nm can be concluded from reports [2], although wavelengths from 243 to 302 nm have been used for
analyses [15]. Maximum absorption (λ max) in aqueous solution occurs at 272/273 nm [4,15,16].
Several analytical methods have been proposed for determination of caffeine in foods.
Gravimetry was the first method developed for this analysis in food products [17]. However, it was
time-consuming, and the poor cleaning procedure of the extract caused overestimation of the
contents [18]. The absorption of electromagnetic radiation in the UV region by caffeine was
described in the early 20th century [19]. A low-cost spectrophotometric analytical method based on
the absorbance at 272 nm was developed in 1948 [20]. The method was faster, simpler and more
accurate than gravimetric methods, but the results could still be overestimated by the presence of
matrix interferents, usually associated with food products [18,21].
High-performance liquid chromatography (HPLC) technique was used in caffeine analysis for
the first time in the early 1970s [22]. Ion exchange chromatography was also used to separate caffeine
from other organic compounds in coffee at that time [23]. The use of HPLC for determination of
caffeine was an important qualitative leap in terms of parameters of precision, accuracy, and speed
[18], also allowing multi-analysis of methylxanthines. The development of stationary phases
containing micro-particles (5 μm) and the use of the gradient system to separate caffeine from other
methylxanthines allowed better resolution in a shorter period of time [18]. However, in coffee
matrix, due to the very low amount of other methylxanthines, HPLC and ultra-high-performance
liquid chromatography (UPLC) isocratic systems have been often applied for determination and
quantification of these compounds, using a reverse-phase column and a mixture of methanol and
water as the mobile phase [2426]. Methods for HPLC analysis evaluating simultaneously caffeine
and additional compounds were developed in the 1980s [27]. In some cases, however, when only
one methylxanthine is present in the matrix, UV-Vis spectrophotometric determination in clarified
extracts may be preferred due to its low cost, fastness and reasonable accuracy and reproducibility
Beverages 2019, 5, 37 3 of 51
when compared to chromatographic techniques [24]. With the development of UV/diode array
detector (DAD), the multi-analysis of methylxanthines improved in terms of precision and accuracy,
since each alkaloid was analyzed at its maximum absorption wavelength [28,29]. A UPLC/DAD
method using a gradient of formic acid 0.1% and acetonitrile has been recently applied for
simultaneous determination of methylxanthines and sugars [30]. Liquid chromatography-mass
spectrometry (LC-MS) is one of the preferred techniques for determining caffeine at low levels
and/or in very complex matrices. The three advantages of LC-MS over conventional HPLC methods
can be represented by three “S”, sensitivity, selectivity and speed. Nonetheless, due to the high cost,
LC-MS techniques are not economically justifiable for the analysis of caffeine in major sources of this
compound, such as coffee [31].
In recent years, spectroscopy in the infra-red region has been introduced as a promising
alternative technique to wet chemical methods [18,32], especially the Fourier transform infra-red
spectroscopy (FTIR) [33]. Methods using this technique dissolve the active principles in chloroform,
followed by filtration of the solution to remove the excipients [18]. Chemometrics is a valuable
mathematical tool that, in combination with different chemical methods, enables the analysis of
many variables in a single sample [18]. Principal components analysis (PCA) has been most
commonly used among chemometric methods to discriminate samples with different chemical
patterns. In the study by Briandet et al. [32], analyses of caffeine were performed by FTIR, followed
by PCA, to discriminate Coffea arabica and Coffea canephora species among lyophilized dry instant
coffee blends from Ireland.
More recently, bare carbon electrodes have been proposed as simple and efficient sensors for
the quantification of caffeine in commercial beverages, presenting similar values when compared to
results from UPLC analyses [30].
3. Contents of Caffeine in Green Coffee Seeds
The caffeine function in coffee plant and seeds seems to be basically related to protection. The
“chemical defense theory” proposes that caffeine in young leaves, fruits, and flower buds acts to
protect soft tissues from predators, such as insect larvae [34] and beetles [35]. The “allelopathic
theory” proposes that caffeine in seed coats is released into the soil to inhibit the germination of
other seeds [4,36].
The genetic factor is the key determinant for caffeine content variation in green (raw) coffee
seeds, especially between species and, to a lesser extent, varieties [37]. The effect of environment,
agricultural practices, including the use of fertilizers, and post-harvest processing on
methylxanthines contents seems to be less important than genetic aspects, except in the case of
secondary processing, such as decaffeination [3840]. The range of caffeine contents reported for
green C. arabica L. vary in the literature between 0.7 and 1.7 g/100 g (dry matter-dm) [15,26,38,4152],
while those for C. canephora vary in the range between 1.4 and 3.3 g/100 g (dm)
[26,38,41,42,45,49,50,5256]. However, common values are in a narrower range, between 1.0 and 1.2
g/100 g, dw, for C. arabica [15,44,4651,54], and between 1.7 and 2.1 g/100 g (dm) for C. canephora
[26,39,43,51,55,56]. In general, C. canephora seeds contain 4070% more caffeine than C. arabica species
[57,58]. C. arabica presents a more homogeneous composition independent of their geographical
origin, due to the low genetic diversity characteristic of the species [41], while higher variability is
observed in C. canephora seeds [55]. The lower caffeine content in C. arabica plants makes them more
vulnerable to phytopathogens, as well as to biological and mechanical stress, than C. canephora plants
[59]. Other methylxanthines have been identified in coffee seeds, but their contents are negligible,
less than 1% of total methylxanthines [41].
Caffeine forms hydrophobically bound π-molecular complexes with chlorogenic acids, the
main phenolic compounds in coffee, especially with 5-caffeoylquinic and 3,5-dicaffeoylquinic acids
[6063], in a 1:1 molecular ratio [64]. According to D’Amelio et al. [62,63], additional complexes may
be formed in green coffee, but at lower rates, with caffeoylquinic acid precursors as well as other
chlorogenic acid compounds, such as feruloylquinic acids, isomers of dicaffeoylquinic acid, and,
during roasting, with chlorogenic acids lactones [65]. Such complexation in green seeds may be one
Beverages 2019, 5, 37 4 of 51
of the facts responsible for the correlation observed between chlorogenic acids and caffeine contents
within the subgenus Coffea [65]. These complexes seem to be also formed in the beverage (see Section
4: Contents of caffeine in roasted coffee seeds and brews).
Regarding post-harvest processing, after the fruits harvest, green coffee seeds are obtained by
one of the different methods from which the most commonly known are called dry, wet and
semi-dry processing [57,66]. Although all methods aim at removing the fruit flesh of coffee cherry,
they do it in different ways, and this affects the final contents of some compounds in the seeds [67].
A few studies have compared the caffeine contents in seeds obtained by different post-harvest
methods. They included comparisons between wet and dry methods [68], wet and semi-dry
methods [48], and dry and semi-dry methods [69]. None of them observed a significant difference in
caffeine results from the different methods.
Regarding the degree of maturation, small variation (about 210%) in the content of caffeine in
seeds have been observed during C. arabica fruits development [42,70]. After fruits development,
significant changes have been observed among seeds from different maturation stages, with a
decrease in contents observed as the fruits ripened and through senescence [44,71].
4. Contents of Caffeine in Regular and Decaffeinated Roasted Coffee Seeds and in Brews
4.1. Caffeine in Roasted and Ground Coffee
The content of caffeine is not significantly altered during coffee roasting due to its thermal
stability, but small losses may occur owing to sublimation. In terms of percent composition, an
increase in caffeine content may be observed due to the loss of thermolabile compounds [57]. The
range of caffeine contents reported for roasted C. arabica seeds vary in the literature between 0.7 and
1.6 g/100 g (dm) [4,15,26,29,50,70,7274], while those for C. canephora vary in the range between 1.8
and 2.6 g/100 g (dm) [29,50,54,72].
4.2. Caffeine in Soluble Coffee
Soluble coffee production typically involves treating ground roasted coffee with hot water and
high pressure to extract the water-soluble compounds, followed by drying. While in Western
countries commercial ground roasted coffee generally consists of C. arabica seeds alone or of a blend
with a small percentage of C. canephora seeds, in some Western countries, a high percentage of C.
canephora or plain C. canephora may be designated for instant coffee production owing to the yield of
higher amount of soluble solids in the brew. This explains the higher caffeine contents often
observed in reconstituted soluble coffees purchased in some countries, such as Brazil, for example
[57]. In C. canephora producing countries, such as Indonesia, for example, this species is also
abundantly used for commercial ground roasted coffee.
Data from a few studies investigating the contents of caffeine in C. arabica and C. canephora seeds
roasted in small scale laboratory roasters, as well as in commercial ground roasted blends and
soluble coffees are presented in Table 1.
Beverages 2019, 5, 37 5 of 51
Table 1. Summary data from studies investigating the caffeine contents in roasted Coffea arabica, and
Coffea canephora seeds, commercial blends and soluble coffees.
Coffee Species/Cultivar/
Type
Country
N
Range of Caffeine
Content
(g/100 g)
Reference
Coffea arabica
cv. Minas
USA
2
0.91.2
[4]
Nr
Ethiopia
4
0.91.1
[15]*
Nr
Hawaii, Colombia, Brazil,
Africa
6
0.80.9
[26]
Nr
Brazil, Mexico, Colombia,
Guatemala
9
1.21.6
[29]
Nr
Croatia
6
0.81.4
[50]
Nr
Brazil
4
0.71.1
[70]
Nr
Brazil
3
1.21.3
[72]
Nr
Poland
2
1.01.1
[73]
cv. Bourbon, Catuaí, Icatu
Brazil
3
1.01.3
[74]
Coffea canephora
cv. Robusta
India, Honduras, Vietnam,
Angola, Caneron
20
2.02.6
[29]
cv. Robusta
Croatia
2
1.82.5
[50]
cv. Robusta
Ivory Coast
1
-
[54]
-
Brazil
3
2.2.2.3
[72]
Commercial blends of C. arabica or C. arabica and C. canephora (cv Robusta or cv. Conilon)
Arabica blends
USA
36
0.8-1.4
[4]
Blend
Africa
1
-
[26]
Blends
Brazil
10
0.81.4
[75]
Blends
USA
6
1.01.6
[76]
Nr
Nr
3
1.41.9
[77]
Blends
Brazil
40
0.81.6
[78]
-
Mexico
3
1.51.7
[79]
Soluble coffee
Nr
Brazil
2
2.02.2
[26]
Nr
Nr
2
4.54.8
[77]
Blends
Brazil
10
1.83.1
[78]
Arabica
Brazil
3
2.84.1
[80]
cv. Conilon
Brazil
3
3.95.8
[80]
Nr
Brazil
9
1.63.2
[81]
Nr
Kenya
2
1.63.4
[82]
Nr
Nr
5
2.23.9
[83]
Note: N: number of samples, Nr: not reported. Analyses performed by liquid chromatography
unless specified by * which indicates analysis performed by UV/vis spectrophotometer.
4.3. Caffeine in Coffee Brew
Caffeine content in coffee brew is closely related to its stimulating properties as well as to about
10% or less of its bitterness [39,47]. The final content of caffeine ingested by coffee drinkers depends
on all factors that affect the seeds contents, including blend composition, which depends mainly on
genetics, the degree of maturation, and the method used to brew the coffee, which may vary
considerably according to social and cultural habits of each country [57]. Caffeine is well extracted
by the most common hot brewing methods [57]. Percolation and decoction methods tend to extract
more than infusion methods which have low extraction power [58], but not only the extraction
method is important. The proportion of water to powder, water temperature, size of particle and
duration of the brewing process are determinant factors [57,76]. Caffeine seems also to be more
easily extracted from dark roasted coffees, and the type of filter may alter the result [84]. Lastly, there
is the variability within the preparation method, which may be very low or quite high in the case of
commercial establishments with different people preparing the brews. In a study that analyzed
caffeine content in 20 different specialty coffees purchased at coffee shops in the United States, the
Beverages 2019, 5, 37 6 of 51
amount of caffeine in brews ranged from 76 to 112 mg/240 mL serving (equivalent to 32 to 47 mg/100
mL) [85]. It was also observed that caffeine content in the same type of coffee purchased in the same
store on six separate occasions ranged from 130 to 282 mg/240 mL serving (equivalent to 54 to 118
mg/100 mL) [85]. Data from studies investigating the contents of caffeine in coffee beverages
prepared by different methods are presented in Table 2. Average values for the most commonly
used methods are presented in Figure 2.
Table 2. Caffeine contents in coffee brews obtained by different extraction methods.
Coffee
Species/
Type
Country
N
Amount of
Powder (%)
to Water/100
mL
Water
Temperature
Brewing
Time
Range of
Caffeine
Contents
(mg/100 mL)
Mean
Caffeine
Content
(mg/100
mL)
Reference
Manual drip
Arabica
Colombia
2
10
100 °C
2 min
48.153.0
50.8
[86]
Arabica
Indonesia
2
10
100 °C
2 min
48.357.9
53.1
[86]
Arabica
Kenya
2
10
100 °C
2 min
54.257.3
55.8
[86]
Arabica
Costa Rica
2
10
100 °C
2 min
50.759.0
57.1
[86]
Arabica
USA
1
7
95 °C
~3 min
-
60.2
[4]
Blend
Brazil
40
10
90 °C
4 min
71.844.1
59.9
[85]
Blend
Brazil
7
10
92.3 °C
Nr
39.451.8
45.6
[87]
Blend
UK
1
Nr
Nr
Nr
-
55.0
[88]
Blend
USA
26
Nr
Nr
Nr
Nr
51.1
[89]
Nr
USA
1
Nr
Nr
Nr
-
52.0
[90]
Nr
USA
14
Nr
Nr
Nr
30.354.8
39.7
[85]
Nr
USA
7
10
100 °C
2 min
Nr
131.0
[91]
Electric dripper
Arabica
Guatemala
1
6
90 °C
6 min
-
100.8
[92]
Arabica
USA
1
7
95 °C
4 min
-
76.5
[4]
Robusta
Vietnam
1
6
90 °C
6 min
-
200.8
[92]
Robusta
Spain
1
6
90 °C
7 min
-
110.0
[93]
Blend
Brazil
40
10
90 °C
4 min
100.9138.3
119.6
[85]
Blend
Spain
2
6
90 °C
7 min
22.0105.0
63.5
[93]
Espresso
Arabica
USA
1
7
90 °C
28 s
-
72.2
[4]
Arabica
Ethiopia
1
14
92 °C
30 s
-
410.0
[94]
Arabica
Ethiopia
1
18
92 °C
25 s
-
420.0
[94]
Arabica
Guatemala
1
17.5
90 °C
24 s
-
100.7
[92]
Arabica
Ethiopia
1
18
92 °C
25 s
-
420.0
[94]
Arabica
Guatemala
1
17.5
90 °C
24 s
-
100.7
[92]
Arabica
Spain
1
18.7
96 °C
21 s
-
209.0
[95]
Robusta
Vietnam
1
17.5
90 °C
24 s
-
177.3
[92]
Robusta
Spain
1
18.7
96 °C
21 s
-
296.0
[95]
Robusta
Spain
1
17.5
90 °C
30 s
-
375.0
[93]
Blend
Scotland
20
Nr
Nr
Nr
160.4650.0
378.0
[96]
Blend
Croatia
4
14
9597 °C
Nr
97.7190.0
142.0
[97]
Blend
Australia
97
Nr
Nr
Nr
58.0700.0
247.3
[98]
Blend
Brazil
40
10
90 °C
28 s
159.5233.2
196.4
[85]
Blend
Spain
2
17.5
90 °C
30 s
63.0241.0
152.0
[93]
Blend
Spain
1
18.7
96 °C
21 s
-
288.0
[95]
Nr
USA
1
Nr
Nr
Nr
-
123.0
[90]
Nr
USA
6
Nr
Nr
Nr
138.3290.2
197.1
[92]
Nr
USA
27
Nr
Nr
Nr
-
250.0
[91]
Regular/Nr
Italy
1
Nr
Nr
Nr
-
317.1
[30]
Lungo/Nr
Italy
1
Nr
Nr
Nr
-
171.3
[30]
Ristretto/Nr
Italy
1
Nr
Nr
Nr
-
533.4
[30]
Mocha
Arabica
USA
1
7
95 °C
10 min
-
92.4
[4]
Arabica
Guatemala
1
8
93 °C
10 min
-
156.4
[92]
Arabica
Ethiopia
1
10
90 °C
Nr
-
128.0
[94]
Robusta
Spain
1
8
93 °C
10 min
-
192.0
[93]
Robusta
Vietnam
1
8
93 °C
10 min
-
226.8
[92]
Blend
Spain
2
8
93 °C
10 min
28.0159.0
93.5
[93]
Beverages 2019, 5, 37 7 of 51
Infusion bag
Arabica
Japan
4
2.5
95 °C
5 min
-
43.2
[4]
French press
Arabica
Guatemala
1
8
98 °C
5 min
-
118.3
[92]
Arabica
Hawaii
4
10
98 °C
6 min
97.0104.9
100.5
[99]
Arabica
USA
1
7
90 °C
5 min
-
75.0
[4]
Arabica
Portugal
1
13
100 °C
2.5 min
84.0106.0
95.0
[99]
Arabica
Ethiopia
1
10
95 °C
5 min
-
52.0
[94]
Robusta
Vietnam
1
8
98 °C
5 min
-
156.4
[92]
Robusta
Spain
1
8
98 °C
5 min
-
136.0
[93]
Blend
Spain
2
8
98 °C
5 min
20.0112.0
66.0
[93]
Aero press
Arabica
Ethiopia
1
6.6
93 °C
1 min
-
78.0
[94]
Cold brewing
Arabica
USA
1
7
95 °C followed
by 10 °C
12 h
-
52.3
[4]
Arabica
USA
1
7
10 °C
12 h
-
51.2
[4]
Arabica
Hawaii
4
10
25 °C
24 h
99.0123.0
112.0
[99]
Arabica
Ethiopia
1
10
25 °C
6 h
-
125.0
[94]
Turkish coffee
Arabica
USA
1
7
90 °C
5 min
-
85.3
[4]
Blend
Croatia
3
8
98 °C
5 min
190.0260.0
233.0
[94]
Boiled coffee
Arabica
USA
1
7
95 °C
5 min
-
85.0
[4]
Arabica
Brazil
3
10
100 °C
Nr
110.8122.5
114.7
[100]
Conilon
Brazil
3
10
100 °C
Nr
171.3192.0
179.8
[100]
Soluble or instant coffee beverage
-
Brazil
10
2
90 °C
-
257.3-280.7
269.0
[85]
-
Nr
2
1
hot water
-
Nr
212
[21]
-
Nr
2
2.2
hot water
-
Nr
74.5
[101]
-
Nr
2
1
hot water
-
Nr
246.8
[102]
-
UK
Nr
Nr
hot water
-
-
39.0
[93]
-
USA
26
Nr
hot water
-
-
44.0
[97]
-
USA
20
Nr
hot water
-
-
39.0
[90]
Read to drink coffee beverage
Nr
Japan
4
-
-
-
-
57.1
[4]
Note: N: number of samples. Nr: Not reported.
Figure 2 illustrates the caffeine content variability among brews obtained by different preparation
methods as well as the variability within each method. The reasons for such values are already aforementioned.
Mostly, in addition to the characteristic power of extraction of the methods, which have been comparatively
consistent in literature reports [78,84], the amount of coffee to water and grid also vary traditionally among
them. It is important to note that these values are given per 100 mL and, in order to compare the consumption
per serving, one has to consider the various existing sizes of cups used for the different types of coffees and
cultures. Usually, however, the more concentrated the brew, the smaller the cup is. This explains the large
servings of infusions of manually dripped coffees (236.5591.4 mL/820 oz) in the USA, in comparison with
1530 mL of a ristretto in Italy.
Espresso
Turkish coffee
Moka
Boiled coffee
Soluble coffee
Electric dripper
French press
Cold brewing
Aero press
Manual drip
Ready to drink coffee beverage
Infusion bag
0
100
200
300
400 253.4
166.5
152.4 126.5 111.8
132.0
99.0 85.0
78.0 59.8 56.5 42. 0
Caffeine contents (mg/100mL)
Beverages 2019, 5, 37 8 of 51
Figure 2. Mean reported caffeine contents (mg/100 mL) in coffee brews obtained from a variety of
blends, and by different preparation methods (values extracted from Table 2).
4.4. Caffeine Complexation with Chlorogenic Acids
It has been reported [62,63] that in addition to the free caffeine content presented in Table 2 and
Figure 2, there are also some complexes formed between caffeine and other compounds. According
to estimates based on results from model complexation with pure compounds and from espresso
beverage analysis [62,63], about 20% of total caffeine in the beverage (considering free and bound
caffeine) forms a complex with 5-caffeoylquinic acid (the main chlorogenic acid in coffee) and, to a
lesser extent, with other major chlorogenic acids and lactones, the latest formed during roasting
process. Structurally explaining, it was proposed, using a model in aqueous solution, that in the case
of complexation with 5-caffeoylquinic acid, and probably also feruloylquinic acid and lactones, the
plane of caffeine molecule would be parallel to the plane of the aromatic ring of the caffeoyl ester
group and that the five and six-membered rings of the nitrogen heterocycle would be equally
involved in the complex formation. In the case of 3,5-dicaffeoylquinic acid, a sandwich structure
(caffeine between two caffeoyl arms) was proposed. For other dicaffeoylquinic acids with lower
chances of complexation, other forms of complexations were proposed. In addition to forming
complexes with chlorogenic acids, according to the authors, about 10% of caffeine could be bound to
other types of substances, such as proteins and, after roasting, to melanoidins. For structures and
other information on chlorogenic acids, please see reference [103].
4.5. Caffeine in Decaffeinated Coffee
Coffee decaffeination is performed in the green seeds but given that recommendations for
residual content are made for roasted coffee, we chose to present this subject here. The first method
for this purpose was developed in Europe, but decaffeinated coffee achieved its first broad market in
the United States during the 1950s [104]. Since the early 1970s, the demand for decaffeinated coffee
has also gradually increased worldwide. World consumption of this type of coffee is difficult to
gauge owing to the lack of separate statistical data for many importing countries. According to the
latest Coffee Drinking Study performed by the National Coffee Association (USA), the consumption
of decaffeinated coffee in 2009 reached 16% of total coffee consumption [104]. Current information
on decaffeinated coffee sales in the US is not available in common coffee market reports. Elsewhere,
consumption of decaffeinated coffee has been fairly static over the last two decades and currently
accounts for approximately 12% of total worldwide coffee consumption [105], although in many
countries new low-caffeine coffee products have been introduced [106].
The decaffeination process can be performed using different methods and extraction vehicles
(water, supercritical carbon dioxide, ethyl acetate, methanol chloride) and often leaves residual
amounts of caffeine in the seeds [57]. According to various national and international rules and
regulations, decaffeination processes should lower the methylxanthine concentration by 9799.9% in
ground roasted coffee and 97% in instant coffee [107109], with no significant influence on other
natural compounds [58]. A more restricted norm is mandatory in the European Community
countries, in which decaffeinated coffee must be 99.9% alkaloid-free [58,107].
About 0.30.5 mg caffeine/100 g have been reported in decaffeinated ground roasted coffees
[76] and 0.70.9 mg/100 g in instant decaffeinated coffees [77]. Regarding decaffeinated coffee brews,
caffeine values from 0.1 to 2.6 mg/100 mL have been reported for brews made from decaffeinated
ground roasted coffees purchased in Brazil and in the United Kingdom [24,88], while in
decaffeinated coffee brews purchased in the United States coffee shops caffeine content reached 18
mg/473 mL serving (~3.7 mg/100 mL) [92]. Contents from 0.9 to 1.2 mg/100 mL have been reported
for reconstituted instant decaffeinated coffee [21,24,88]. It is noteworthy mentioning that, for a heavy
coffee drinker, the ingestion of multiple servings of decaffeinated beverages could result in caffeine
doses equivalent to a caffeinated beverage [110].
5. Global Caffeine Consumption Through Coffee
Beverages 2019, 5, 37 9 of 51
Currently, approximately 80% of the worlds population consumes a caffeinated product every
day [111], mainly for its stimulating effects, which makes it the most widely consumed psychoactive
substance in the world [112116]. So where does coffee consumption stand in relation to other
caffeine sources? Over the last 50 years, world coffee consumption increased at a mean annual
growth rate of 1.9%, to almost 9.7 million tons in 2018 [117]. The highest coffee consumption occurs
especially in the Americas, Europe and Japan [117]. The European Union is responsible for the
largest consumption volume (about 28% of the total world consumption), but breaking up into
individual countries, the USA are the first consumer country (about 16% of the total world
consumption), followed by Brazil (the largest producer, with 13% of world consumption), European
Union countries, and Japan. Philippines, Russia, Ethiopia, Canada and Mexico contribute about
2.8%, 2.5%, 2.4%, 2.3% and 1.5% of total world consumption, respectively [117]. In Canada (6.5
kg/year coffee per capita consumption), as well as in many European countries, such as Denmark
(8.7 kg/year), Norway (9.9 kg/year) and Finland (12 kg/year), coffee consumption is very prevalent
and accounts for most of the daily caffeine consumption among adults [118].
Considering these data, it is not difficult to accept that coffee is the major contributor to caffeine
intake in most countries worldwide. Exceptions occur in specific areas, such as regions of South
America [119,120], Ireland and the United Kingdom, China, India and other Asian countries [121],
where other beverages, such as maté leaves, natural guaraná beverage, and black/green Camelia
sinensis teas, are typically consumed.
However, additional sources of caffeine and other methylxanthines, mainly theobromine, are
also consumed worldwide and, for this reason, over the past years, there has been a renewed interest
in understanding caffeine exposure in populations. The daily caffeine intake and the type of
caffeinated product consumed vary considerably around the world, with sex, age segments, cultural
habits and household income. The age interval considered for each category also varies among
studies and reviews. The cultural habits influence not only the total intake but also the percent
contribution of foods to such intake. Differences also occur among geographic regions and between
metropolis and countryside, where certain types of products are not available. Other than cultural
habits, one reason for the differences in consumption levels is the variable concentrations of caffeine
found among home-made beverages [122].
Regardless of the longstanding consumption of caffeine-containing beverages in the diet and
acknowledging all the aforementioned aspects, there is a lack of current and comprehensive
population-based data on caffeine intake. Most studies on intake still cite information dating back
the 1980s and 1990s, when Barone and Roberts [90] highlighted results from earlier
population-based surveys or data from the food balance sheets published by the Food and
Agriculture Organization (FAO) of the United Nations in 1995 [123], which contains average food
consumption estimates in all world countries for all genders and ages. However, the national
consumption of caffeine summarized in the food balance sheets depended heavily on official
statistics, which were thought to be greatly unreliable at that time [111]. In addition, the estimates
did not include soft drinks, although they may be a major source of caffeine for children and
adolescents, especially in Western society [90,111,121,124]. As a result, the intake was
underestimated. More recently, in a survey performed in 2015 by Mitchell et al. [125], including
37,602 interviews in the United States, adults consumed, on average, 152 mg of caffeine/day and the
elderly 207.3 mg of caffeine/day. Less was consumed by adolescents (83 mg/day) and children (30
mg/day). Due to the high consumption of regular soft drinks, they accounted for the main source of
caffeine among children and adolescents, while coffee was the main source of caffeine consumption
in adults. This was also observed in an earlier study performed by the same authors in 2014 [124].
According to Mitchell et al. [125], the American population, considering all ages, consumes 164.5 mg
caffeine/day, with coffee being responsible for 64% of this amount. Considering only adults and the
elderly, the coffee contribution was reported to be 63% and 76%, respectively.
Further, in 2015, the Comprehensive European Food Consumption Database organized by the
European Food Safety Authority (EFSA) [126] was used for the calculation of caffeine intake from all
sources. It contains data from 39 surveys in 22 different European countries for a total of 66,531
Beverages 2019, 5, 37 10 of 51
participants. In most European surveys covered by EFSA’s Food Consumption Database [126],
coffee was the predominant source of caffeine for adults and the elderly, contributing, on average
(considering all countries), 78% and 84% of total caffeine intake, respectively. Exceptions were
Ireland and the United Kingdom, where tea was the main caffeine source. The average caffeine
consumption by adults in the UK was reported to be 138 mg/day for adults and 165 mg/day for the
elderly, with the coffee contribution of only 34% and 33%, respectively. Nordic countries were
among the largest consumers. The reported intakes by adults in Denmark, Finland, and Sweden
were 320, 236, and 205 mg/day for adults, and 362, 214, and 222 mg/day for the elderly, respectively,
with coffee contributions of 88%, 94% and 85% for adults and 91%, 97% and 89% for the elderly,
respectively.
There is no official data on caffeine consumption in Latin American countries [127]. However, it
is believed that caffeine intake in countries, such as Brazil and Argentina, are high due to the large
popular intake of coffee and maté tea. A similar situation may occur in China due to the high intake
of green tea [127]. Recently, in Brazil, caffeine consumption was estimated [128] based on 13,569
interviews by the national consumption survey (Consumer Expenditure Survey, 20082009)
performed by the Brazilian Institute of Geography and Statistics. The average daily caffeine intake
by Brazilians of all genders, age groups, geographic locations, and income classes was, on average,
about 130 mg/day, with coffee being responsible for 90% of daily caffeine intake. Considering adults
and the elderly, total intakes were, on average, 137 and 146 mg/day respectively, with coffee
contributions of 90% and 95%, respectively.
Regarding pregnant or lactating women, studies that evaluated the available data on daily
caffeine intake among these groups are very scarce [118]. Using data collected in the United States
by Share of Intake Panel for pregnant women, Knight et al. [129] estimated in 2004 the average daily
caffeine consumption from beverages in about 58 mg/day. Later, in 2009, using data from
interviews, Olmos et al. [130] estimated daily caffeine consumption among Argentinean pregnant
women in approximately 200 mg/day.
6. Caffeine Metabolism
The first studies on coffee bioactivity in humans date back centuries ago. Coffee was present in
medical journals since the 18th century, and the early physiological explanations already
emphasized nervous and vasomotor stimulation [131]. Caffeine is involved in several biological
effects in the human body, most of them related to improvements in brain and Central Nervous
System (CNS) functions. However, the biological effects associated with caffeine consumption
largely depend on its biotransformation in the body [132]. The earliest reported study on caffeine
metabolism seems to have been carried out in 1850 by Lehmann, who, following an oral dose, failed
to detect traces of caffeine in human urine. This was even before the chemical characterization of the
compound by Hermann Emil Fischer (18521919) in 1882 [114]. Albanese, in 1895 was the first to
report that dogs and rabbits fed large doses of caffeine excreted a monomethylxanthine, showing
that ingested methylxanthines were demethylated. A few months later, Rost published data on the
excretion of unchanged caffeine and theobromine in a cat, a dog, a rabbit, and a man [114]. The most
complete pharmacokinetic studies on caffeine in human subjects as well as various experimental
animals started to be performed late in the 1950s [133137]. Other studies in humans followed
including minor methylxanthines, accessing their primary metabolites in plasma [138141] or in
urine [141143]. Paraxanthine, 1-methylxanthine, and 1-methyluric acid have been the first urinary
metabolites of caffeine identified in men and rabbits [133]. Subsequently, theobromine and
theophylline were also reported to be urinary metabolites of caffeine in men and dogs [135,144].
Later, improved analytical methodologies confirmed the extensive metabolization of caffeine,
identifying secondary metabolites such monomethylxanthines (1-methylxanthine and
7-methylxanthine), and methyluric acids (1-methyluric acid and 1,7-dimethyluric acid) in both
plasma [145148] and urine [148,149]. However, the most important results obtained in earlier
studies have been ratified by recent studies.
Beverages 2019, 5, 37 11 of 51
6.1. Absorption
The pharmacokinetics of caffeine is relatively well elucidated and has been reported in several
studies, including recent ones [114,146,148,149]. Most existing reports on the matter come from
studies using pure caffeine in the form of solutions, capsules, and tablets [138,140,150,151]. Among
the food matrices evaluated, the most common are coffee, cola, and cocoa products
[114,131,140,141,146148]. The bioavailability of caffeine is similar among dogs, rats, and mice
[114,152]. Because no hepatic first-pass effect complicates its pharmacokinetics, caffeine absorption
from foods and beverages has been shown to be independent of the administration route, age, sex,
health status, and concomitant administration of alcohol, drugs and nicotine exposure [58,114,132].
Following ingestion, caffeine is rapidly and almost completely (up to 99%) absorbed into the
bloodstream. About 20% absorption occurs in the stomach, and the remaining 80%, in the small
intestine [114,138]. Caffeine can also be quickly absorbed through the oral mucosa [153], as it does
not need to pass the stomach and intestine to get into the blood [153156], and when administered
via enema [146]. The pharmacokinetics of a similar dose of caffeine after a single administration of a
coffee enema (107.2 ± 2.2 mg) versus coffee consumed orally (96.3 ± 1.3 mg) was compared in healthy
male subjects (n = 11). The relative bioavailability of caffeine obtained from the coffee enema was
about 3.5 times lower than when coffee was consumed orally [132,146].
In different studies, the time to reach the peak plasma concentration (Tmax) after oral doses of 72
to 375 mg of caffeine in healthy adult volunteers has most often varied between 15 and 60 min
[138,140,142,147,155,156], but, in some circumstances, it has taken 120 min after oral intake, mainly
due to delayed gastric emptying [153,157,158]. This involves motility of gastrointestinal tract,
individual physiology, and vehicle (the type of food matrix, volume, solid or liquid, capsule, gum)
[112,159]. In studies using different food matrices, the absorption of caffeine from soda and
chocolate was slightly delayed, relative to coffee [112,140,159], caffeine in a chewing gum format was
absorbed faster than in coffee [153] or in capsules [154] and in capsules, the absorption of the same
caffeine dose was faster than in coffee [112,140].
After oral consumption of 70500 mg of caffeine, peak plasma concentration (Cmax) varies in
reports between 1.1 to 17.3 µg/mL [114,138,140,141,145,147,148,155]. Because caffeine is usually
ingested in a daily diet, owing to accumulation, its actual plasma concentration may be more than
the value reported in single-dosage studies, which are usually preceded by wash out or clearance
period [58,132].
6.2. Metabolism and Distribution
After absorption, caffeine is quickly distributed to most tissues (mean volume distribution of
0.61.0 L/kg) and body fluids (i.e., bile, milk, saliva, semen, sweat, and urine) [114], although it is
received in the body as a xenobiotic substance. Studies have reported that concentrations of caffeine
in saliva are approximately 2040% lower than in plasma [145,160163]. The limited plasma protein
binding (estimated at 1730%) combined with the relatively hydrophobic properties of caffeine
allow its passage through all biological membranes [14,114] and enables it to easily cross
intracellular barriers, including placental (mother-fetus-mother) and blood-brain barriers [114,132].
The distribution pattern usually does not change during a person’s entire life, and it can be
significantly higher in women when compared to men [164,165]. However, severely obese subjects
have exhibited an increased volume of distribution [165,166], although this volume was decreased
with weight reduction [114]. The effect was more important in females, and it was suggested that
caffeine distribution into the adipose tissue was incomplete, representing 7080% excess of body
weight in obese subjects [164]. Caffeine and its major metabolite paraxanthine can be found in the
amniotic fluid throughout gestation [167], and they are distributed to fluids and tissues of the fetus
[167]. Caffeine has also been identified in women’s milk [133,168].
Caffeine is rapidly and extensively metabolized in the liver cells to form dimethyl and
monomethylxanthines, dimethyl and monomethyluric acids, and uracil derivatives [114]. Most of
the metabolism of caffeine and other methylxanthines is performed by phase I (cytochrome P450
CYP) enzymes, mainly CYP1A2, a major enzyme, among P450 enzymes in the human liver, that
Beverages 2019, 5, 37 12 of 51
accounts for approximately 13% of the total content of this enzyme group. The activity of the
CYP1A2 isoform accounts for almost 90% of caffeine metabolism [114,143,169]. The remaining
pathways are related to CYP1A1, CYP2E1, CYP2A6, as well as mono-oxygenase and
N-acetyltransferase activities [58,132].
Paraxanthine is the major caffeine metabolite in plasma, while methylated xanthines and
methyluric acids are the main metabolites excreted in urine [114,148]. The metabolic pathway of
caffeine in humans is shown in Figure 3. From the metabolic pathways of caffeine, it is apparent that
each metabolite may be derived from more than one precursor [139].
Figure 3. Metabolic pathways of caffeine and metabolites in humans. Grey arrows indicate lower
production of the metabolite. Adapted from [132]. XO: xanthine oxidase, NAT 2: N-acetyltransferase
2.
The initial metabolization by CYP1A2, begins with 3-demethylation of caffeine, resulting in the
formation of 1,7-dimethylxanthine (paraxanthine), which represents about 84% of primary caffeine
metabolites [143,170] (Figure 3). CYP1A2 enzyme may also convert caffeine to theobromine (~12%)
[171]. CYP2E1 participates in the metabolism of caffeine accelerating the synthesis of theobromine
and theophylline (~4%) by 7- and 1-demethylation [58]. In a lesser extent, caffeine may be converted
to 1,3,7-methyluric acid [132]. Paraxanthine, the major primary caffeine metabolite may be
demethylated by CYP1A2 to form the main metabolite, 1-methylxanthine (~70%), which may be
oxidized to 1-methyluric acid by xanthine oxidase. Secondary metabolites, such as 7-methylxanthine
(~20%) may also be oxidized to 7-methyluric acid [58,114], and paraxanthine may also be
hydroxylated by CYP2A6 to form 1,7-dimethyluric acid or acetylated by N-acetyltransferase 2, to
form 5-acetylamino-6-formylamino-3-methyluracil, an unstable compound that may be
non-enzymatically deformylated to form 5-acetylamino-6-amino-3-methyluracil [14,132,172] (Figure
3).
The pharmacological and biochemical properties of caffeine make it a model substrate capable
of revealing activity of other drug metabolizing enzymes in animals and humans [173,174]. Caffeine
has been extensively used as a probe to assess the metabolic activity and phenotyping of CYP1A2,
CYP2A6, N-acetyltransferase 2 and xanthine oxidase enzymes activities, providing valuable
information on cancer susceptibility, drug interactions and toxicity in population studies of healthy
Beverages 2019, 5, 37 13 of 51
subjects, given that these are important detoxifying enzymes [175]. For example, the ratio of
paraxanthine to caffeine, or the ratio of paraxanthine plus 1,7 dimethyluric acid to caffeine in plasma
has been used as an indicator of CYP1A2 activity [58,176,177].
The half-life of caffeine in plasma seems to vary, on average, between 2.55 h in adults [14],
although larger variations from 2.3 to 12 h have been reported [156], indicating substantial
intersubject variability in caffeine elimination time [138]. Caffeine’s half-life is altered in the neonatal
period. It increases shortly after birth due to lower activity of cytochrome P-450 enzymes and the
relative immaturity of some demethylation and acetylation pathways [178]. Caffeine’s half-life is
about 80 h for the full-term newborn infant and can be over 100 h in premature infants [179]. Infants
up to the age of eight to nine months still present a reduced ability to metabolize caffeine, excreting
in urine about 85% of the administered caffeine in its unchanged form [180]. Moreover, caffeine’s
half-life may be influenced by other factors, including sex, smoking habit, use of oral contraceptives,
and specific biological moments, such as pregnancy [14,114]. Caffeine’s half-life has been reported to
be 2030% shorter in females than in males [178] and 30 to 50% shorter in smokers compared to
nonsmokers’ adult males [181]. On the other hand, caffeine’s half-life is almost doubled in women
taking oral contraceptives [182,183] and greatly prolonged (up to 15 h) during the last trimester of
pregnancy [184] and in patients with liver diseases [14]. In fact, health status, in general, is another
factor that influences caffeine metabolism. The biotransformation is related to the proper function of
the liver and kidneys, and the decrease of caffeine plasma clearance is a typical complication of these
organ’s diseases [132]. Cirrhosis and non-cirrhotic cases of viral hepatitis are the most common liver
diseases that may disturb such process [58]. Obesity significantly increases plasma half-life, and
decreases elimination rate, without significant effect on the clearance [132].
The consumption of high amounts could lead to saturation in caffeine metabolism [148]. Thus,
while linear pharmacokinetics have been observed with caffeine intake between 70 and 100 mg,
doses ranging between 250 and 500 mg have resulted in increased plasma concentration, a
non-linear kinetic and prolonged half-life [148,154].
Regarding the interindividual variability in caffeine metabolization speed, the activity of CYP
enzymes has been reported to vary between individuals up to 50-fold for some metabolic reactions
[185]. The large interindividual variability of CYP1A2 activity may be due to factors, such as gender,
race, genetic polymorphisms, and environmental influences, such as smoking or exposure to
chemicals [186,187]. For example, higher activity of CYP1A2 has been shown in men compared to
women [169,177,188190] and in white compared to black subjects [191]. The enzyme activity is
increased by cigarette smoking [192], and by moderate daily coffee consumption (at least three cups
of coffee). Some studies have reported that herbal medicines can induce or inhibit human CYP1A2
activity [193196]. During pregnancy, the excretion of 1-methylxanthine and 1-methyluric acid was
decreased in women of between 3436 gestational weeks that consumed caffeine doses from 123 to
369 mg [197], owing to a decrease in CYP1A2, xanthine oxidase, and acetyltransferase activities
[198]. CYP1A2 is also inhibited by oral contraceptives [186].
CYP1A2 appears to be polymorphically distributed in human populations. The CYP1A2*1F
polymorphism characterizes the so-called “slow metabolizer” phenotype, which decreases enzyme
activity and inducibility and allows caffeine to stay longer bioavailable [192]. Conversely,
homozygous individuals (AA) for the allele CYP1A2*1A are rapid caffeine metabolizers. These
individuals present lower caffeine plasma levels and shorter exposure to this compound [192]. On
the other hand, homozygous individuals (CC genotypes) have been considered as “slow
metabolizer” [199,200]. Such polymorphism was observed in 1999 by Sachse et al. [192]. They first
sequenced intron 1 of the CYP1A2 gene in DNA from eight volunteers and observed only one
polymorphism represented by an AdenineCytosine (AC) substitution at position 734
(CYP1A2*1F) in the CYP1A2 gene. After this result, a mutation-specific test was developed, and a
functional significance of this polymorphism was assessed in 185 healthy Caucasian non-smokers
and in 51 smokers by genotyping and phenotyping using caffeine (100 mg/oral doses). The A variant
(CYP1A2*1A) was more frequent (46%, n = 108) followed by the A/C genotype (44%, n = 104), and
almost 10% (n = 24) corresponded to C variant (CYP1A2*1B). The authors observed a significant
Beverages 2019, 5, 37 14 of 51
difference between the A/A and A/C genotypes in the 5h plasma paraxanthine/caffeine ratios, but
not between the A/C and the C/C genotypes and indicated that the A allele is a recessive factor for
high inducibility. Differences among CYP1A2 fast and slow metabolizers still require further
investigation. The importance given to the phenotype of individuals in relation to CYP1A2 activity
using plasma, saliva or urine samples [177] is increasing, since it has been observed that this
characteristic may influence the metabolism of individuals and may or not make them susceptible to
developing certain diseases. For example, slow caffeine metabolization can enable the occurrence of
side effects exposure [200] or make individuals more susceptible to hypertension development [201].
Few studies investigated whether genetic polymorphisms have an effect on coffee and caffeine
consumption. Rodenburg et al. [202] studied the effect of CYP1A2*1F polymorphism and smoking
on coffee intake in 6.689 subjects in the Netherlands. Smokers drank almost one cup of coffee (0.90
cup/day ~ 110 mL) per day, more than did non-smokers. A meta-analysis [203] of 47,341 individuals
of European ancestry, including five studies within the United States, was performed using directly
genotyped and 2.5 million single nucleotide polymorphisms (SNP). Two sequence variants were
found to be significantly associated with increased coffee consumption: rs2472297-T, located
between CYP1A1 and CYP1A2 at 15q24 (P = 5.2 × 1014) and rs6968865-T, near AHR at 7p21 (P = 2.4 ×
1019). An effect of 0.2 cups a day per allele was observed for both SNP. According to the authors,
possibly rs2472297-T and rs6968865-T allow people to consume more coffee because in these carriers
clearance of caffeine is more effective as a result of higher CYP1A1 or CYP1A2 enzymatic activities.
In agreement with this result, Tantcheva-Poór et al. [204] and Djordjevic et al. [169] observed that
heavy coffee consumers have higher CYP1A2 activity than those drinking less coffee, whereas
Carrillo and Benitez [205] observed that low CYP1A2 activity we more often found in subjects with
toxic symptoms linked to caffeine consumption.
A more recent meta-analysis included 12 studies and looked at the association between habitual
coffee intake and CYP1A2 rs762551 polymorphism that splits the population in AA (rapid caffeine
metabolizers), AC and CC genotypes (slow caffeine metabolizers). The analysis showed an
association between the AA genotype and coffee consumption [OR = 1.13, 95%, CI = 1.031.24, P =
0.06]. This association was found in men, individuals younger than 59 years, and Caucasians, but not
in females, individuals older than 59 years, and Asians [206].
6.3. Excretion
Despite caffeine efficient penetration in tissues and fluids, there is no long-term accumulation
of it or its metabolites in the body as seen by whole-animal autoradiography using radiolabeled
caffeine [114,207] and in humans [114]. Various experimental and human studies have proved that
caffeine is excreted mostly via kidneys. In humans, the total urinary excretion of
monomethylxanthines (1-methylxanthine, 3-methylxanthine, and 7-methylxanthine), dimethylurate
derivates (1,3-methyluric acid and 1,7-methyluric acid) and monomethylurates (1-methyluric acid),
has been estimated to be equivalent to about 9095% of the amount of caffeine orally administrated
(57.5 mg of caffeine/kg body weightbw), and that less than 5% is recovered as caffeine itself
[114,208]. After peaking, plasma concentrations of caffeine decrease more rapidly than those of its
metabolite paraxanthine. Therefore, despite important interindividual differences, the concentration
of paraxanthine becomes higher than that of caffeine within 810 h after administration [114,132].
Caffeine clearance is strongly dependent on renal blood flow and urine passage because this
alkaloid and its primary metabolites are extensively reabsorbed (98%) in renal tubule, but the final
urine concentration significantly correlates with the plasma caffeine level, as well as fluid intake
[114]. The fecal excretion is not so relevant because it covers only a small percentage (15%) of the
caffeine ingested [114]. The microbial products identified in human feces are: 1,7-dimethyluric acid
(44%), 1-methyluric acid (38%), 1,3-dimethyluric acid (14%) and 1,3,7-trimethyluric acid (6%), and
caffeine (2%) [114,209].
7. Health Benefits of Caffeine Consumption
Beverages 2019, 5, 37 15 of 51
The most well-known acute effects of caffeine consumption are stimulation of brain function
and improvement in mood, and physical performance [57]. However, along the past few years,
several epidemiological studies have associated moderate coffee consumption with the reduction in
the relative risk of development of chronic degenerative diseases and death [210219], and caffeine is
one of the compounds responsible for many of these benefits. They include reduction in the risk of
Parkinson’s and Alzheimer’s diseases as well as hepatoprotective effects. The mechanisms involve
antioxidant and anti-inflammatory activities, among others. The main caffeine benefits will be
commented below.
7.1. Caffeine, Mood, and Behavior
Once caffeine is absorbed, it exerts a variety of pharmacological actions at diverse central and
peripheral sites [220,221]. These effects are predominantly related to its antagonistic activity at
adenosine receptors [222], which are widely distributed throughout the body, allowing the
substance’s wide range of effects. Of the four adenosine receptors (A1, A2A, A2B, and A3), caffeine acts
mainly as an antagonist to adenosine A1 and A2A receptors, that are expressed in the CNS [112,223].
In humans, A1 and A2A have been shown to be activated in normal plasma caffeine concentrations
(1050 µM), while the other two receptors (A2B and A3) are only stimulated at higher concentrations
[223]. Thus, when caffeine intake is able to cause an extracellular concentration of 1050 µM, it
selectively blocks adenosine receptors and competitively inhibits the action of adenosine [224].
Consequently, caffeine increases the responses from dopaminergic receptors [225] and the release of
various neurotransmitters, such as norepinephrine, dopamine, and serotonin [226], stimulating
psychomotor properties and improving behavioral functions, such as mood and wellbeing [227],
sense of energy [228], and effects related to alertness, mental focus/attention [229,230], memory,
speed at which information is processed [115,230], awareness, and reaction time [227,229,231].
According to EFSA [126], who reviewed all existing evidence of caffeine on mental performance,
generally, a dose of 75 mg is needed to obtain these effects, although very large differences in
individual responses to caffeine are observed as stated above. There is a consensus that in most
people, at low (~50250 mg) to moderate doses (~250-400 mg) [123,231] or 15 mg caffeine/body
weight/day for a 70-kg person [223] positive changes occur in mood and human behavior, such as
enhanced energy, well-being, sociability, willingness and motivation to work, improved
self-confidence and cognitive function, including enhanced alertness and mental focus, vigilance,
learning and memory [123,232]. This is generally true for both caffeine-deprived and
caffeine-tolerant individuals [223].
7.2. Caffeine and Exercise Performance
Caffeine exerts a positive effect on endurance and exercise capacity owing to the
aforementioned neural mechanisms that trigger a chain of physiological reactions, which makes it
an ergogenic resource [233]. Exercise performance is shown to be significantly improved by oral
caffeine administration or by the consumption of dietary sources, either by avoiding fatigue,
improving substrates supply or by enhancing oxygen uptake [221,234]. Ergogenic effects of caffeine
are similar in both non-habitual and habitual caffeine consumers [235] and have been observed after
administration of doses between 3 and 6 mg/kg of body weight [233,236]. Considering a 70 kg
individual and a supplemented dose of 3 mg/kg of caffeine, the intake could be estimated in 210 mg
[237]. Caffeine also increases coordination [238] and reduces the perception of pain and fatigue
[222]. Because caffeine increases metabolic rate, energy expenditure, lipid oxidation and presents
lipolytic and thermogenic activities, all favorable components for weight management, coffee has
been used for weight loss [239,240].
Historically, in 1984 the International Olympic Committee had caffeine on the list of banned
substances for urinary concentrations greater than 15 μg/mL (equivalent to 5–6 mg of caffeine/kg
bw), being considered a doping infraction [241]. In 2003, the World Anti-Doping Agency (WADA)
included caffeine in the list of stimulants banned from sports competitions, with a maximum
allowed urinary concentration of 12 μg/mL [242]. However, it was observed that both the commonly
Beverages 2019, 5, 37 16 of 51
consumed doses and the supplementation doses indicated to promote ergogenic effect (3 to 6 mg/kg
bw) resulted in urinary concentrations far below the limit proposed by WADA. Because of the
difficulty to differentiate the low levels of habitual caffeine ingestion from the intentional use of
caffeine to improve athletic performance, WADA removed caffeine from the list of prohibited
substances in 2004 and added it to its monitoring program [243], which includes substances that are
not prohibited in sport, but which WADA examines in order to detect patterns of misuse [237].
7.3. Caffeine and Antioxidant and Antiinflammatory Activities
Some of the beneficial health effects reported for caffeine have been associated with antioxidant
properties [244250] although not all studies have found such activity at physiological micromolar
concentrations [251]. Caffeine has been reported to be an efficient scavenger of hydroxyl radicals
generated by the Fenton reaction [239], evaluated by electron spin resonance spin trapping, and it
has also been reported to inhibit lipid peroxidation of rat liver microsomes at millimolar
concentration by reducing the production of TBARS (thiobarbituric acid reactive substances) and
lipid hydroperoxides [252]. Caffeine metabolites, especially 1-methylxanthine,1-methyluric acid
[251], and 1-methylurate [253] have also exhibited effective in vitro antioxidant activity, in the case
of 1-methylxanthine, equivalent to ascorbic acid activity. Caffeine intake has also been responsible
for an increase in glutathione levels in rats [250]. Corroborating in vitro results, the average plasma
iron-reducing capacity of human subjects was higher after regular coffee consumption than after
decaffeinated coffee consumption [254], suggesting that whole coffee is more efficient than
decaffeinated coffee in respect to its antioxidant capacity. This was later confirmed by other studies
[255,256].
Since inflammation is correlated with and influenced by various cytokines and chemokines,
reduction of these markers should decrease the degree of overall inflammation [257]. The
anti-inflammatory action of caffeine is thought to be related to phosphodiesterase inhibition and/or
with adenosine receptor antagonism mechanisms [244,258]. Caffeine anti-inflammatory potential
has also been linked to modifications in cell signaling molecules production [259]. In many studies,
caffeine potentiated the release of anti-inflammatory cytokines, including interleukin 10 (IL-10)
[260,261]. Additionally, caffeine mediates immune-suppression of pro-inflammatory cytokines
release, including tumor necrosis factor alpha (TNF-α), interleukin 2 (IL-2) and interferon-gamma
(IFN-γ), which have a central role in autoimmune disease initiation and propagation [262,263].
7.4. Caffeine and Antimicrobial Activity
Regarding caffeine’s antimicrobial activity, there are a few in vitro studies showing that
caffeine contributes to the antibacterial effect of coffee against Streptococcus mutans, the main
cariogenic bacteria [264], as well as intestinal pathogenic bacteria’s [265,266]. Additionally, a study
showed the effectiveness of caffeine in inactivating and inhibiting significantly the growth of
Escherichia coli O157:H7 in brain heart infusion broth, indicating that caffeine has potential as an
antimicrobial agent for the treatment of E. coli O157:H7 infection and could be investigated further as
an eventual food additive to increase bio-safety of consumable food products [267].
7.5. Caffeine and Neurodegenerative Diseases
The effect of caffeine on neurodegenerative diseases has gathered considerable attention in the
last years [244]. Several studies have reported that regular coffee/caffeine intake is related to lower
risk of neurodegenerative diseases development, especially Parkinson’s and Alzheimer´s
[245,268272], and prevention of memory decline during aging [273,274]. A few findings from
selected studies and meta-analyses are presented here.
Parkinson’s disease is characterized by selective degeneration of dopaminergic neurons in the
midbrain with a clinical presentation of motor and non-motor symptoms and by the prominent
alpha-synuclein-containing proteinaceous inclusions, called Lewy bodies [275,276]. Coffee
consumption appears to reduce the risk of Parkinson’s disease or to delay its onset [238], by
Beverages 2019, 5, 37 17 of 51
attenuating dopaminergic neurodegeneration [277]. In a meta-analysis of 26 studies from the USA,
Europe, and Asia, a 25% lower risk of Parkinson’s disease was found in coffee drinkers as compared
to non-coffee drinkers [237,271]. The overall risk has been reported to fall at least by 2432% per 300
mg (three cups of 100 mg) increase in caffeine intake [271]. Higher risk reduction (up to 80%) have
been suggested for the intake of more than four cups of caffeinated coffee daily [231]. This inverse
relationship was confirmed by similar findings in two larger ethnically diverse cohorts involving
47,351 men and 88,565 women. In both studies, the consumption of caffeinated (but not
decaffeinated) coffee was associated with reduced risk of developing Parkinson’s disease [277,278].
Differences between genders [278] regarding the association of caffeine intake and the risk of
Parkinson’s disease have been reported: while for men a strong inverse association was found, for
women, this association was non-linear (“U-shaped”), with the lowest risk occurring at moderate
intake (one to three cups/day). The authors further investigated this difference in two large cohorts
in the USA [279,280] and found an interaction among the use of postmenopausal hormones, caffeine
intake, and risk of Parkinson’s disease. In one of the cohorts, the risk was increased among women
who were in hormonal replacement therapy [279]. In the second cohort, the risk was increased
among hormone users who were heavy coffee drinkers (more than six cups/day) [280]. The reason
for the interference of estrogens on the protective effect of coffee was not clear [279,280]. Additional
studies involving caffeine protection against Parkinson´s disease development are presented in
Table 3.
Alzheimer’s disease is the most frequent cause of dementia, leading to a progressive cognitive
decline [237]. Definitive diagnosis of Alzheimer’s disease is based on the presence of senile plaques
and neurofibrillary tangles that are identified in post-mortem brain specimens [281]. The formation
of Alzheimersdisease-specific lesions is attributed to the pathological accumulation of either toxic
extracellular amyloid beta peptide in the brain [282], or intraneuronal hyperphosphorylated Tau
protein [283]. Constituents of the lesions are prone to promote synaptic deficits leading to memory
impairments [284]. Currently, there is no medication against Alzheimer’s disease once it is installed
[285], but there are a few ways of preventing it, among them the consumption of foods rich in
polyphenols and caffeine [281]. A meta-analysis has reported an inverse association between
regular coffee consumption and the development of Alzheimer’s disease, with a 27% risk reduction
observed in the highest category of coffee consumption compared with the lowest [286]. The
mechanism for caffeine protection is believed to be related to an anti-inflammatory effect on the A1
and A2 receptors as well as to the reduction in the deposits of toxic beta-amyloid peptide in the
brain [287]. Alzheimer’s disease mouse model study confirmed these findings, reporting that heavy
coffee intake (the human equivalent of 500 mg caffeine or five coffee cups/day) was able to protect
against and could treat Alzheimer’s disease [287].
Exceptionally, in a study evaluating Japanese-American men, the authors did not find a
significant association between caffeine intake and the risk of dementia [288]. However, they
interestingly reported that, at autopsy, patients in the highest quartile of caffeine intake (>277.5
mg/dayall caffeine sources) were less likely to have any of the neuropathological lesions, such as
Alzheimer’s disease-related lesions, ischemic microvascular lesions, cortical Lewy bodies,
hippocampal sclerosis or generalized atrophy [288]. Additional studies on the protective effect of
caffeine against Alzheimer’s disease are presented in Table 3.
7.6. Caffeine and Liver Diseases
In the past three decades, caffeine has been related to a lower incidence of chronic liver diseases,
such as cirrhosis and hepatocellular carcinoma [289291]. Additionally, in several studies, regular
coffee consumption has been significantly associated with reduced hepatic fibrosis related or not
with non-alcoholic fatty liver disease [292] and with chronic hepatitis C [293]. In 2016, The
International Agency on Research on Cancer (IARC) evaluated several studies [294] and concluded
that higher coffee consumption was associated with lower blood concentrations of biomarkers of
liver damage, including alanine aminotransferase and γ-glutamyl transferase [295]. Moreover, in
prospective studies, coffee consumption was associated with a lower risk of liver cirrhosis [296].
Beverages 2019, 5, 37 18 of 51
Two important meta-analyses that combined data from cohort and case-control studies, adjusting
the results for potential confounders (age, sex, alcohol intake, smoking, and history of liver diseases),
confirmed the inverse association between coffee consumption and liver cancer [297,298].
Caffeine, together with other bioactive compounds, such as chlorogenic acids and trigonelline,
has been reported to be responsible for coffee hepatoprotective effect. The mechanisms underlying
the potential benefits of caffeine have not yet been fully determined. However, some plausible
explanations have been suggested [291]. In patients with chronic hepatitis C, a suggested possible
mechanism would be the alteration in liver signaling and inflammatory pathways [291]. In a rat
model, caffeine suppressed connective tissue growth factor expression by interfering with a
profibrogenic cytokine, transforming growth factor beta (TGF-β) signaling through the Smad
pathway [299]. Smad comprises a family of structurally similar proteins that are the main signal
transducers for receptors of TGF-β, which are critically important for regulating cell development
and growth. Caffeine also has a direct inhibitory effect on hepatic stellate cells by downregulating
focal adhesion kinase and actin synthesis and also induces hepatic stellate cells apoptosis [300].
Additional studies on the protective effect of caffeine on the liver are presented in Table 3.
Table 3. Complementary studies on the protective effect of caffeine against neurodegenerative and
hepatic diseases.
Subjects’
Origin
Sample
Study Type
Conclusions
Reference
Neurodegenerative diseases
Finland
n =
6.710
Large prospective study
(follow-up for 22 years)
Results support the hypothesis that coffee
consumption reduces the risk of Parkinson’s disease.
[301]
Greece
n =
26.173
Population-based
prospective cohort
(follow-up for 35 years)
Results support the hypothesis that coffee
consumption reduces the risk of Parkinson’s disease.
[302]
Denmark
n =
1.876
Large case-control study
Moderate coffee intake (3.15 cups/day) was associated
with a lower odds ratio for Parkinson’s disease.
[303]
Mostly from
USA, Europe,
and Asia
-
Dose-response
meta-analysis
Non-linear relationship between coffee intake and the
risk of Parkinson’s disease was found, with maximum
protection effect at approximately 3 cups/day and no
improvement after that.
[304]
Canada
n =
6.434
Prospective analysis of
risk factors (follow-up
for 5 years)
Daily coffee consumption decreased the risk of
Alzheimer’s disease by 31% during the follow-up.
[305]
Finland
n =
1.409
Population-based cohort
study (follow-up for 21
years)
Coffee drinkers at midlife had a lower risk of dementia
and Alzheimer’s disease later in life compared to those
who drank no or little coffee, with lower risk (65%
decreased) in those who drank 35 cups per day.
[269]
Mostly from
USA, Europe,
and Asia
-
Systematic review
Findings indicate that moderate coffee/caffeine intake
decreases the risk of cognitive impairment/decline and
dementia/Alzheimer’s disease later in life.
[306]
Liver diseases
Mostly from
USA, Europe,
and Asia
-
Systematic review and
dose-response
meta-analysis
Regular intake of three cups of caffeinated and
decaffeinated coffees was associated with reductions of
27% and 14% in the risk of hepatocellular carcinoma,
respectively.
[307]
USA, France
and Brazil
-
Systematic review and
meta-analysis
Decreased risk of advanced liver fibrosis and liver
inflammation among hepatitis C virus-infected
patients who consumed caffeine on a regular basis.
[308]
USA
n =
5.944
Population-based cohort
study
Doseeffect relationship between coffee and caffeine
consumption and decrease in aminotransferase (ALT)
levels, reducing the prevalence of above-normal ALT
value by 50% for two cups/day, and by 66% for three
[309]
Beverages 2019, 5, 37 19 of 51
cups/day.
Italy
n = 732
Population-based study
Coffee caffeine may inhibit the onset of alcoholic and
nonalcoholic liver cirrhosis.
[310]
8. Potential Adverse Effects of Caffeine Consumption
8.1. Caffeine Acute and Chronic Toxicity
Based on scientific evidence, moderate caffeine consumption is currently considered by EFSA
[126], Food and Drug Administration (FDA) [311] and the Scientific Committee on Food within the
European Commission (SCF) [312], among other health authorities, to be a safe habit. However,
caffeine acute toxicity effects related to excessive intake may occur and are well characterized. The
first studies on the toxicity of caffeine were performed in the 19th century, initially with animals
[313], and soon after, with humans [314]. In 1850, Lehmann [314] reported several adverse
symptoms after acute oral administration of 2 to 10 g of caffeine. Such intoxication results in
‘caffeinism’, which refers to a syndrome characterized by a range of adverse reactions, for instance,
restlessness, nervousness, anxiety, irritability, agitation, muscle tremor, insomnia, headache,
diuresis, tachycardia, arrhythmia, pulse irregularity and increased frequency, elevated respiration
and gastrointestinal disturbances (e.g., nausea, vomiting, diarrhea) [315]. Besides tachycardia and
diuresis, caffeine toxicity in children has also been implicated in severe emesis, photophobia,
palpitations, muscle twitching, convulsions, and unconsciousness, especially at doses around 80
mg/kg of body weight [14].
Regarding lethality, caffeine death-related reports in humans are unusual, implying rather
significant concentrations which are not provided by regular coffee drinking. Only a few cases have
been reported in the literature [316]. Concerning the exact dose, as previously mentioned, the
metabolism and physiological effects of caffeine vary greatly, depending on several factors. For slow
metabolizers, the lethal dose will be lower than for fast metabolizers. Foods or medications taken
simultaneously will affect caffeine metabolism and, therefore, also the lethal acute dose [316]. Other
complications would be age and previously existing cardiovascular diseases or other types of
diseases. According to Frerichs [317], severe symptoms might be induced in humans about 15 min
after oral administration of 25 g of the drug, which is an extremely high amount. Currently, studies
using potentially lethal doses (LD) are not performed in humans, and the perception of LD is only
based on extrapolation of animal studies or case reports. Tarka and Cornish [318] determined the
LD50 (the amount given acutely which causes death of 50% of the animals) for caffeine in rats and
extrapolated the results to humans. According to them, the LD 50 for caffeine in humans would be
about 192 mg/kg of body weight. For Cappelletti et al. [245] the LD of caffeine should be about 10
g/day, which according to the authors could be comparable to drinking 100 cups of instant coffee.
Arnaud [14] also estimated in 10 g the acute LD of caffeine which they extrapolated to 150200
mg/kg bw in agreement with Tarka and Cornish [318], but death has been reported after ingestion of
6.5g of caffeine [14], which is in agreement with a report from Kerrigan and Lindsey [319] suggesting
lethal doses to be typically in excess of 5 g, although according to the authors, there has been several
cases in which adults and teenagers consumed between 510 g and survived. The survival of a
patient who supposedly ingested 24 g of caffeine has also been reported [315,320].
In general, high chronic exposure to caffeine (more than 400600 mg/day) has also been
associated with a range of dysfunctions involving the gastrointestinal, liver and renal systems,
besides musculature [315,320], unstable bladder, mainly developed in women [321]. In more
extreme cases, symptoms may include myopathy, hypokalemia, muscular weakness, nausea, vomit,
diarrhea, and weight loss [322].
Following, the most commonly reported potential negative effects of acute and chronic caffeine
consumption on the human body are discussed, especially the effects on CNS and behavior,
cardiovascular system, glucose metabolism, bone and calcium balance, reproductive and
development effects and carcinogenesis. Caffeine withdrawal syndrome will also be approached.
8.2. Potential Adverse Effects of Caffeine on Mood, Behavior, and Sleep
Beverages 2019, 5, 37 20 of 51
The effects of caffeine on mood in adults have been extensively studied and the most common
negative effects reported after caffeine intake are related to its stimulating effects due to the
aforementioned responses in dopaminergic D1 and D2 receptors [225] and release of
neurotransmitters, such as norepinephrine, dopamine, and serotonin [112]. Although ingestion at
low to moderate doses tends to licit the pleasant sensations previously described in this review,
higher doses consumed either on a single occasion or within short periods of time can produce or
exacerbate jitteriness, insomnia (especially in those who are caffeine-abstinent) [112], nervousness
and anxiety, especially in those with preexisting psychiatric anxiety disorders, but also in healthy
adults, particularly when they are non-habitual caffeine consumers [232]. The dose range considered
to cause anxiety and mood change varies considerably among authors and official guidelines, from
400 to 2000 mg caffeine/day [14,232,323325]. In slow metabolizers, such negative effects can be felt
at much lower doses compared to fast metabolizers, as low as 50 mg or less. After repeated intake,
however, tolerance to general effects of caffeine is usually observed. The mechanism to increase
tolerance is not well understood and is highly variable among the population, but it has been
attributed to upregulation of adenosine receptors [112]. However, in adults, tolerance to such
anxiogenic effect develops with frequent caffeine consumption, even in genetically susceptible
individuals [326]. The inter-individual variability in the anxiogenic response to caffeine intake has
been suggested to be caused by a single nucleotide polymorphism in the gene coding for the
adenosine A2A receptor (ADORA2A) [326,327]. In any case, such high doses are consumed only by a
small segment of caffeine consumers, and individuals experiencing the anxiogenic effects of caffeine
as well as slow metabolizers who are sensitive to its effects in general are likely to avoid the use of
the substance [315]. Thus, the self-limiting nature of caffeine intake reduces caffeine potential to
produce anxiety in adults [326,327].
Regarding sleep, in adults, single doses equivalent to about 100 mg of caffeine or more (1.5
mg/kg bw/day in a 70 kg adult) have increased sleep latency, in a dose-dependent manner and
reduced sleep duration when consumed close to bedtime [328]. This may be accompanied by
impairment of sleep quality, characterized by an increased number of spontaneous awakenings and
body movements. Doses lower than 100 mg do not appear to have such an effect on sleep in most
people [14,329]. Chronic high consumers of caffeine, however, are less likely to report sleep
disturbances than individuals consuming caffeine more occasionally, also suggesting the
development of tolerance to the effects of caffeine on this parameter [330].
There seem to be only a few human intervention studies, meta-analysis and controlled trials
investigating the effects of caffeine on psychological, behavioral, cognitive functions and sleep of
children and adolescents [331342]. In an earlier study by Rapoport et al. [333], daily caffeine
consumption was investigated for two weeks in relation to self-reported anxiety, parents/teachers’
ratings of children’s behavior and side effects in pre-pubertal children. Data from 19 children were
analyzed depending on whether they were “low” (up to 50 mg/day) or “high” (more than 300
mg/day) caffeine consumers. The results provided evidence that “high” habitual caffeine consumers
(and their parents) tended to report more side effects during the caffeine withdrawal period,
compared to “low” caffeine consumers, suggesting the development of tolerance and withdrawal
symptoms in “high” habitual consumers. Moreover, in “high” habitual caffeine consumers, despite
the significant improvements in tasks related to vigilance and significant increases in locomotor
activity, symptoms as “nervous/jittery” were also reported, although not classified as anxiety
[126,333]. Considering the existing literature data on the subject up to 2015, according to EFSA [126],
regular consumption of caffeine (up to about 3 mg/kg bw/day, approximately 60120 mg of
caffeine/day) does not appear to induce behavioral changes in children and adolescents.
8.3. Potential Adverse Effects of Caffeine on Cardiovascular System
Investigations regarding the effects of caffeine consumption on the cardiovascular system
generally focus on evaluating the heart functioning as the onset of morbi-mortality covering the
main risk factors. For this, it is especially important to distinguish acute from long-term
cardiovascular effects of caffeine [126,172]. Single 200250 mg doses of pure caffeine acutely
Beverages 2019, 5, 37 21 of 51
increased plasma renin activity, catecholamine concentrations, and blood pressure, and were able to
induce cardiac arrhythmias (mostly atrial) in healthy subjects [343,344].
Possible mechanisms for the acute cardiovascular effects of caffeine include antagonistic action
on adenosine receptors, activation of the sympathetic nervous system (release of catecholamines
from adrenal medulla), stimulation of adrenal cortex (release of corticosteroids), renal effects
(diuresis, natriuresis, activation of the renin-angiotensin-aldosterone system), and inhibition of
phosphodiesterase’s (increase in cyclic nucleotides), although the contribution of each of these
mechanisms to the acute cardiovascular effects of caffeine is unclear [126,201], and may depend on
the source of caffeine, the dose administered, and on plasma concentrations prior to caffeine
administration [126].
According to epidemiological data, the coffee effect on blood pressure differs with CYP1A2
genotype [345]. While in fast caffeine metabolizers (CYP1A2*1A) the effect of caffeine on blood
pressure seems to be insignificant, in slow metabolizers (CYP1A2*1F) the hypertensive effect seems
to prevail [345]. However, it has been observed that in fast metabolizers, caffeinated cola
consumption, but not coffee, has been associated with hypertension, which may be due to the lack of
polyphenols in the cola beverages [345]. Watanabe et al. [346] examined the blood pressure-lowering
effect of chlorogenic acids (the main polyphenols in coffee) in patients with mild hypertension
through a placebo-controlled, randomized clinical trial. Subjects (n = 28) were randomized to receive
treatment with chlorogenic acids (140 mg/day) from green coffee extract or placebo daily for 12
weeks. In the chlorogenic acids group, but not in the placebo group, blood pressure (systolic and
diastolic) decreased significantly during the ingestion period, demonstrating the hypotensive action
of coffee phenolic compounds, a mechanism that seems to involve nitric oxide-mediated
vasodilation [347].
Comparing the hypertensive effect with frequency of caffeine consumption, acute increases in
systolic and diastolic blood pressures, as well as in pulse pressure have been reported after single
doses of caffeine ranging from 80 to 250 mg, in coffee abstainers and in habitual caffeine consumers,
after 12 to 48 h withdrawal [126,348350]. Although the hypertensive effect of caffeine was observed
in many repeated-dose studies, it was not as consistent as in the acute-dose studies [344]. Tolerance
usually develops within a couple of days, and it is accompanied by a reduced release of adrenaline,
noradrenaline, and rennin, compared with the non-tolerant state. Although fast tolerance
development has been observed in habitual coffee drinkers (within one to three days), the
hypertensive response is regained after relatively brief periods of abstinence (12 h) and depends on
how much caffeine is consumed, the schedule of consumption, and on the half-life and elimination
of caffeine from the body [351,352].
Caffeine intake has also been associated with the occurrence of arrhythmias in humans. It
produces a direct stimulation of myocardial tissue, leading to increased heart rate and force of
contraction [14]. Cardiovascular disease has been assessed by a range of outcome variables,
including death from myocardial infarction or Coronary Heart Disease (CHD), non-fatal myocardial
infarction or coronary event, angina pectoris and/or hospitalization for CHD. Prospective
epidemiological studies and case-control studies were more likely to show an increased risk in
cardiovascular disease onset only when five or more cups of coffee were consumed per day (≥ 500
mg caffeine/day) [353357]. However, as with blood pressure, in patients with a history of
cardiovascular disease, slow caffeine metabolizers (CYP1A2*1F), were associated with increased risk
of myocardial infarction when coffee consumption was increased [199]. Additional studies on the
effect of caffeine on blood pressure and cardiovascular diseases are presented in Table 4.
8.4. Potential Adverse Effects of Caffeine on Glucose Metabolism and Insulin Resistance
Coffee consumption has been associated with reduced risk of type 2 diabetes [358364].
However, a key issue that remains to be resolved is whether the consumption of caffeinated and
decaffeinated coffees is similarly associated with the reduced risk of type 2 diabetes [365,366].
Shearer et al. [365] observed that glucose infusion rates and measures of whole-body metabolic
clearance were greater in rats that received decaffeinated coffee (2 g/100 mL) than in placebo or
Beverages 2019, 5, 37 22 of 51
caffeine (20 mg/100 mL) added to decaffeinated coffee (2 g/100 mL), indicating increased
whole-body insulin sensitivity in decaffeinated coffee. It was concluded that caffeine can antagonize
the beneficial effects of decaffeinated coffee. Most evidence, in fact, indicates that caffeine alone
promotes adverse effects on glucose metabolism [366368] and reduces insulin sensitivity [364,369].
The intake of about 500 mg/day by usual coffee drinkers was associated with higher average
daytime glucose concentrations and exaggerated post-prandial glucose responses in diabetic
patients [366]. In healthy individuals, the ingestion of caffeinated coffee with either a high or a low
glycemic index meal significantly impaired acute blood glucose management and insulin sensitivity
compared with decaffeinated coffee [367]. These effects could be partially explained by the direct
inhibition of glucose uptake in adipocytes and skeletal muscle through antagonism of adenosine
receptors [370] or possibly as a result of elevated plasma epinephrine [371].
On the other hand, both caffeinated and decaffeinated coffee consumption enhanced insulin
sensitivity in a cross-sectional study [372] and in epidemiological studies [373376], suggesting
beneficial effects of both drinks on glucose homeostasis. Other studies have found that when
controlling for total coffee intake, caffeine intake was not associated with diabetes risk [363,364],
showing that the consumption of both decaffeinated e and caffeinated coffees is significantly
associated with a lower risk of developing type 2 diabetes [358,359,366,373,374]. As with blood
pressure, although most data from short- and long-term studies indicate that caffeine intake
promotes adverse effects on glucose metabolism [377379], evidence points out that coffee
components other than caffeine, especially chlorogenic acids and trigonelline [102,365,380], exert
several positive effects on glucose homeostasis, balancing caffeine effects [364,378382].
In conclusion, although regular consumption of both decaffeinated and regular coffees is
proved to reduce the risk of diabetes, decaffeinated beverages seem to be more beneficial for
glycemic control than caffeinated ones, although most epidemiological studies have failed to
discriminate the effects of these two types of coffee on the protective effect of coffee against type 2
diabetes. Additional studies on this topic are presented in Table 4.
8.5. Potential Adverse Effects of Caffeine on Calcium Balance
This is one of the most discussed potential adverse effects of caffeine intake. Caffeine potential
to adversely influence calcium excretion and bone metabolism was investigated by epidemiological
studies which evaluated the relationship between caffeine intake and the risk of fracture and fall
[383387], bone mineral density (BMD) and osteoporosis [388393], and the effect on calcium
homeostasis [394,395]. On the potential risk factor for bone fracture and fall, most studies reported a
lack of association between caffeine intake and increased risk of fracture considering consumption
below 400 mg/day [383387]. However, in a cross-sectional study, caffeine intake was associated
with an increased incidence of low trauma fractures [396]. When the consumption was estimated in
more than 544 mg caffeine/day, consumers had a higher risk of hip fracture than those who ‘almost
never’ consumed coffee. Hallström et al. [397] also reported that a daily intake of ≥ 330 mg caffeine
might be associated with a modestly increased risk of osteoporotic fractures (relative risk (RR): 1.20,
CI: 1.071.35), especially in women with low calcium intake. However, when stratified by calcium
intake, the increased risk was only significant when calcium intake was low (less than 700 mg/day).
No trend in increased risk of osteoporotic fractures was observed with higher caffeine intake in
participants with high calcium intake.
Caffeine consumption of 175 mg/day has been positively associated with increased 24-h urinary
calcium excretion [398]. Heaney and Rafferty [394] also reported that acute consumption of
caffeinated beverages (6092 mg caffeine) produced small increases in calcium excretion, which,
according to the authors, could be offset by small increases in calcium intake (1530 mL of milk).
Based on a study group of women who habitually consumed low-calcium, Ribeiro-Alves et al. [395]
reported that exposure to 285 mg caffeine resulted in increased calcium excretion. However, in
another study by Barger-Lux et al. [399], when a greater amount of caffeine was ingested by healthy
premenopausal women for a prolonged time (400 mg/person/day for 19 days), no effect on calcium
absorption, endogenous fecal calcium or urinary calcium excretion was found, despite the
Beverages 2019, 5, 37 23 of 51
observation of bone remodeling. Interpretation of caffeine’s effects on bone metabolism is complex
since caffeine intake is usually associated with other risk factors for osteoporosis, such as lower
calcium intake [393,400] and advanced ages [401,402]. Considering all available data, and evaluating
the same population of postmenopausal women studied by Barger-Lux et al. [399], a model was
elaborated: coffee intake higher than 1000 mL/day (760 mg caffeine/day) could induce excess
calcium loss, while intakes of 150300 mL coffee/day (112224 mg caffeine/day) would have little
impact on calcium balance [403].
The effect of coffee consumption on BMD in elderly men and women, with regards to the
CYP1A2 genotype, was recently evaluated by Hallström et al. [397]. A decrease in BMD of the
proximal femur was observed in men consuming four or more cups of coffee daily. It was also found
that, in high coffee consumers, fast caffeine metabolizers had lower BMD values than slow
metabolizers. Considering that in a higher CYP1A2 activity condition caffeine is more rapidly
metabolized and the concentrations of its metabolites in plasma increase in relation to the parent
compound, it was suggested that the deleterious effects of coffee consumption on bone might be due
to caffeine metabolites, especially paraxanthine [391]. This metabolite has been found to be a potent
suppressor of transforming growth factor beta (TGF-β) in vitro [404], which stimulates bone
formation [405]. Moreover, deactivation of the adenosine receptors, which are expressed in bone
cells, can result in reduced bone formation [406]. Since paraxanthine acts through the same
mechanism as caffeine, that is the competitive antagonism interaction with A1 and A2 adenosine
receptors [405], it could also act on such signaling pathway to reduce bone formation. Therefore,
paraxanthine seems to be the main contributor to the coffee effect on BMD reduction, being the rapid
metabolizers of caffeine at higher risk for bone loss induced by coffee than slow metabolizers [391].
Considering all previously published data, Nawrot et al. [232] concluded in a review published
in 2003 that caffeine intake lower than 400 mg/day does not have significant effects on bone status or
calcium balance in individuals ingesting at least 800 mg calcium/day. No other recommendations
were made thereafter. Additional studies on this topic are presented in Table 4.
8.6. Potential Adverse Effects of Caffeine on Fertility and Reproductive and Developmental Effects
The effects of caffeine consumption have been reviewed in terms of reproduction or fertility
[407410] and in terms of pregnancy outcomes, including spontaneous abortion, birth weight,
gestational length, and congenital malformations [410]. Consistent relationships between caffeine
intake and subfecundity have not been observed to date. However, bringing together existing data
available up to 2003, Nawrot et al. [232] suggested that the consumption of caffeine at doses greater
than 300 mg/day might reduce fecundability in fertile women. In respect to male fertility, Dlugosz
and Bracken [407] suggested that doses higher than 400 mg/day might decrease sperm motility
and/or increase the percentage of dead spermatozoa, but not sufficiently to affect the male fertility in
an adverse manner.
Once pregnant, women who regularly consume caffeine may be at risk of miscarriage, but
current evidence of spontaneous abortion remains insufficient to allow conclusions regarding the
potential role of caffeine [411]. The existing data do not support convincing results that caffeine
consumption increases the risk of any perinatal adversity [412]. Nevertheless, based on several
studies evaluating the association of caffeine intake and risk of perinatal adversities, Nawrot et al.
[232] advised that women who are pregnant or are planning to become pregnant should limit the
consumption of caffeine to less than 300 mg/day, which is historically a common amount consumed
by this group with no adversities [413].
The potential adverse impact of caffeine consumption during pregnancy on fetal growth has
also been a concern for many years. Caffeine increases the levels of cyclic adenosine monophosphate
through inhibition of phosphodiesterase’s, which might interfere with fetal cell growth and
development [414]. It is known that caffeine ingested by the mother is rapidly absorbed from the
gastrointestinal tract and readily crosses the placenta, being distributed to all fetal tissues, including
the CNS. Once present in the fetus organism, caffeine has increased half-life due to the immaturity of
the enzyme complex involved in its metabolism. Therefore, if a pregnant woman does not limit her
Beverages 2019, 5, 37 24 of 51
caffeine intake, the fetus and neonate may be exposed to substantial amounts of caffeine and
metabolites and may suffer the consequences of potentiated adverse effects [14]. According to
Rosenberg et al. [415], no association was found between drinking caffeine-containing beverages at
levels up to 400 mg caffeine/day and five types of malformation (inguinal hernia, oral clefts, cardiac
defects, pyloric stenosis, neural tube defects) in a case-control study. These results were ratified by
Olsen et al. [416]. Although published results are not yet entirely consistent, evidence suggests that
caffeine intake at doses higher than 300 mg/day may cause adverse effects on some fetus
developmental parameters, such as fetal intrauterine growth retardation or decrease in birth weight
[407,417]. Being cautious, based on two prospective cohort studies that investigated positive
association between caffeine intake during pregnancy and risk of adverse birth weight-related
outcomes [418,419], EFSA [126] concluded that caffeine intake from all sources up to 200 mg/day by
pregnant women in the general population does not raise safety concerns for the fetus. The
association between caffeine intake and other adverse pregnancy-related outcomes was less
consistent [419]. Additional studies on this subject are presented in Table 4.
8.7. Potential Carcinogenicity of Caffeine
In 1983, a safety assessment on caffeine consumption was performed by the SCF within the
European Commission [312]. Comparatively high doses of caffeine had shown weak teratogenic
effects in experimental animals and mutagenic effects in vitro, but not in vivo, and it was concluded
that there was no evidence for concern over carcinogenic, teratogenic, or mutagenic effects of
caffeine in man at the actual levels of intake (between 2.0 and 4.5 mg/kg of body weight/day) and
that human epidemiological studies provided no evidence for any association between coffee
consumption and congenital defects [312]. In 1987, caffeine underwent another extensive review in
which the FDA declared its safety for all consumers, including children. In 1991, several studies
suggesting the potential carcinogenic effects of coffee, specifically regarding bladder [420422] have
led IARC [423] to classify coffee as possibly carcinogenic to humans, based on limited evidence of
association with cancer of the urinary bladder from case-control studies, and inadequate evidence of
carcinogenicity in experimental animals. However, IARC [423] concluded that there was no
evidence for concern over carcinogenic, teratogenic, or mutagenic effects of caffeine in man at the
observed levels of intake (between 2.0 and 4.5 mg/kg/day) and that human epidemiological studies
provided no evidence for any association between coffee consumption and congenital defects. Most
evidence indeed supports a lack of substantial relation between caffeine intake, as measured by
coffee consumption, and various types of cancer, including gastric cancer [424], renal cancer [425],
breast cancer [426,427] and colorectal cancer [428]. In 2016, IARC [294] re-evaluated studies
investigating the association between coffee consumption and cancer. For this re-evaluation, a much
larger database of prospective cohort and population-based case-control studies that controlled
adequately potential confounders, including tobacco and alcohol consumption, was available. For
bladder cancer, there was no consistent evidence of association with coffee drinking. In several
studies, relative risks were increased in men but were null or decreased in women, consistent with
residual confusion caused by smoking or occupational exposures among men. IARC concluded that
positive associations between coffee and bladder cancer reported in some studies could have been
due to inadequate control for tobacco smoking, which can be strongly associated with heavy coffee
drinking, and, as a result of the re-evaluation, IARC changed coffee classification to no
carcinogenicity to humans [294]. In the same evaluation, for endometrial cancer, the five largest
cohort studies showed mostly inverse associations with coffee drinking. These inverse associations
were also observed in cohort and case-control studies of liver cancer in Asia, Europe, and North
America, in several types of studies, which lead IARC in 2016 to acknowledge the protective effect of
coffee [294], and suggest that an increase in consumption of one cup of coffee/day (and
consequently, an increase in caffeine intake of about 50150 mg/day) was associated with reduced
risk of kidney, breast, buccal and pharyngeal, colorectal, endometrial, esophageal, leukemic,
pancreatic, and prostate cancers [429431] and, therefore, regular caffeine intake does not seem to be
Beverages 2019, 5, 37 25 of 51
associated with increased risk of cancer when considering whole coffee consumption. Additional
studies on this topic are presented in Table 4.
8.8. Caffeine Withdrawal Syndrome
It has been widely experienced that the sudden cessation of regular caffeine ingestion produces
specific interrelated symptoms [432,433], which are named caffeine withdrawal syndrome as stated
by the Diagnostic and Statistical Manual of Mental Disorders of the American Psychiatric
Association (APA) [434]. Characteristic symptoms of caffeine-withdrawal include headache,
drowsiness, lethargy, fatigue, work difficulty (decreased motivation for work and impaired
concentration), decreased wellbeing (including decreased self-confidence and increased irritability),
fall in blood pressure and rise in cerebral blood flow [432,433,435]. These are opposite sensations to
those obtained after caffeine consumption. Withdrawal symptoms generally begin about 12 to 24 h
after cessation of caffeine consumption and reach a peak after 20 to 48 h. However, in some
individuals, these symptoms can appear within only 3 to 6 h and can last for a week [316]. Thus,
even a short abstinence equivalent to missing the morning cup of coffee can lead to significant
unpleasant effects [436,437]. The syndrome is probably specifically due to the discontinuation of
caffeine intake because it persists in spite of analgesic consumption [112] and is reversed by caffeine
ingestion [438]. The fact is that daily consumption of caffeine can result in physical dependence and
the removal of caffeine causes withdrawal symptoms, irrespective of the pattern of intake across the
day [435] and of small (129 mg: one to two cups of coffee) or large (2548 mg: 2030 cups of coffee)
amounts of caffeine ingested [432,437,438]. Caffeine withdrawal syndrome can be avoided if caffeine
ingestion decreases slowly.
Slow caffeine metabolizers are less likely to experience sedation on withdrawal or onset of
anxiety on resumption. This is in accordance with the general principle that slow reduction
minimizes withdrawal symptoms [439]. Moreover, slow metabolizers are likely to drink less coffee
and less frequently, which also decreases withdrawal syndrome probability.
Withdrawal symptoms have also been reported in newborns whose mothers were heavy coffee
drinkers during pregnancy. These infants displayed irritability, high emotivity, and even vomiting.
Symptoms begin at birth and spontaneously disappear after a few days [440]. Caffeine withdrawal
may also occur in children who largely consume soft drinks [441].
Despite the described withdrawal symptoms, according to the Diagnostic and Statistical
Manual of Mental Disorders, (by APA) [434] caffeine is not present in the category of substances
classified as causing “substance dependence”, since the substance does not cause the severity of
withdrawal or harmful drug-seeking behaviors as street drugs or alcohol and these symptoms are
easily and reliably reversed by ingestion of caffeine.
Table 4. Complementary studies on the potential adverse effects of caffeine on health.
Sample
Type of Study
Coffee/Caffeine
Doses
Conclusions
Reference
Anxiety
Healthy subjects
(n = 13)
Clinical study
9 mg of
caffeine/kg body
weight (bw)
High caffeine doses (> 400
mg/day) significantly increased
some anxiety aspects.
[442]
Subjects with panic
disorders (n = 28),
performance social
anxiety disorder
(n= 19), and healthy
subjects (n = 26)
Clinical study
480 mg of caffeine
Patients with panic disorders
and performance social anxiety
disorders had a higher number
of induced panic attacks, some
specific anxiety symptoms, and
a more severe anxiety response
than healthy volunteers.
[443]
Cardiovascular system
Normotensive
Critical review and
Single doses
Increase in systolic blood
[345]
Beverages 2019, 5, 37 26 of 51
subjects
meta-analysis
200250 mg of
caffeine
pressure by 314 mm Hg and
diastolic blood pressure by 413
mm Hg.
Hypertensive
subjects
Systematic review
and meta-analysis
Single doses
200300 mg of
caffeine
Induced average increases of 8.1
mm Hg in systolic and 5.7 mm
Hg diastolic blood pressure,
observed within the first 60 min
after intake and persisting for
180 min, on average.
[444]
Hypertensive
subjects
Systematic review
and meta-analysis
Doses ≥ 410 mg of
caffeine/day for at
least 7 days
No effect on heart rate
associated with consumption as
compared to those who
consumed < 410 mg/day.
[445]
Glucose metabolism and insulin resistance
Healthy subjects
Systematic review
and meta-analysis
36 mg of
caffeine/kg bw
Acute caffeine ingestion reduces
insulin sensitivity.
[446]
People with type II
diabetes
Systematic review
of randomized
controlled trials
~200500 mg of
caffeine
Caffeine intake increased blood
glucose concentrations by
1628% of the area under the
curve (AUC) and insulin
concentrations by 1948% of the
AUC.
[447]
Bone and calcium balance
Healthy
postmenopausal
women
(n = 205)
Randomized
clinical trial
280420 mg of
caffeine/day
Daily consumption of 280420
mg caffeine/day may accelerates
bone loss from the spine and
total body in women with
calcium intakes below the
recommended dietary allowance
of 800 mg.
[448]
Healthy women (n =
61.433)
Longitudinal
population-based
≥ 560 mg of
caffeine/day
Not associated with higher rate
of fractures and bone mineral
density
[385]
Healthy women
Systematic review
and meta-analysis
28 cups of
coffee/day
The fracture risk was 14% higher
in women and 24% lower in men
with the highest level of coffee
consumption.
[387]
Fertility and reproductive and developmental effects
Healthy women
Systematic review
and dose-response
meta-analysis
≥600 mg of
caffeine/day
Coffee/caffeine consumption is
associated with a significantly
increased risk of spontaneous
abortion.
[449]
Healthy pregnant
women
Systematic review
up to 300 mg of
caffeine/day
Lack of birth defects following
consumption of caffeine in
healthy pregnant women.
[354]
Carcinogenesis
Healthy subjects
Dose-response
meta-analysis
13 cups of
coffee/day
Caffeinated but not
decaffeinated coffee
consumption was negatively
associated with basal cell
carcinoma risk.
[450]
Beverages 2019, 5, 37 27 of 51
Healthy subjects
Meta-analysis
5 cups of
coffee/day
Caffeinated but not
decaffeinated coffee consumtion
might have chemo-preventive
effects against malignant
melanoma risk, coffee.
[451]
9. Maximum Caffeine Intake Recommendations
Because of the high caffeine consumption worldwide, continuous research on potential health
effects and on safety aspects has been performed. However, there is currently no recognized
reference standard for caffeine consumption, such as an acceptable daily intake (ADI) [452]. A
number of assessments have been made around the world, and exposure limits have been adopted
for different population groups. Some of the most important ones are presented in Table 5.
Individuals that do not consume caffeine daily are at greater risk of negative physiological
effects than the habitual consumers [127]. In the same way, for those who are highly sensitive to the
stimulating effects of caffeine, it is hard to determine the safety limit for its consumption [453]. Some
slow metabolizers called popularly “ultra-sensitive”, can be over-stimulated by the delay in caffeine
metabolization and clearance in the body. Moreover, individual differences in responses to caffeine
may occur not only at the metabolic (pharmacokinetic) level but also at the drug-receptor
(pharmacodynamic) level and they can contribute to the quality and magnitude of direct
physiological effects and as a consequence to caffeine consumption. Additional factors are age, use
of other drugs and circadian factors [122]. The biological mechanisms of these possible sources of
variation involve interactions at multiple sites with the enzymes that break down caffeine in the
liver, as well as receptors in the brain that are affected by caffeine [454]. For these reasons, the exact
amount of caffeine necessary to produce adverse effects varies from person to person depending on
their sensitivity to caffeine [453] and therefore caffeinated beverages should be consumed by these
individuals with caution, until a person understands how it interacts with his/her particular genetic
structure and health profile [455]. If willing to consume caffeine, ultra-sensitive consumers should
try small amounts until they find the amount appropriate or acceptable for them, offering the
wellbeing sensation without causing side-effects. For these people, decaffeinated coffee, which still
contains residual amounts of caffeine, may be more appropriate.
Commenting on adolescents and children, a study from The University Children’s Hospital in
Zurich showed that caffeine can interfere with children and teenagers (aged 1016 years, n = 32)
sleep, possibly hindering proper brain development, and, therefore, limiting caffeine intake was
recommended [456]. Moreover, recently, EFSA recommended no consumption of caffeine for
children under 12 months [126] and this involves no consumption of all caffeine-containing drinks
and foods, such as chocolate drinks, maté, Camelia sinensis teas, and soft drinks, which are often
offered by parents.
Attention deficit hyperactivity disorder (ADHD) is one of the most common children’s mental
health conditions. It involves symptoms of inattention or impulsivity and hyperactivity that lead to
behavioral impairments [457]. Many studies have investigated the possible role of caffeine in
ADHD. In an animal model study using rats, caffeine restored the function of dopamine as a
neurotransmitter in the brain [457]. It is also known that being a vasoconstrictor, caffeine can mimic
ADHD medications, such as amphetamine, that also constricts blood vessels and increase
concentration [458,459]. Notwithstanding the fact that caffeine appears to be beneficial for some
children (and also adults) with ADHD, lack of adverse effects is not guaranteed. Overconsumption
should be avoided in children, especially on a regular basis and over a long period of time. It is
noteworthy mentioning that in some Latin American producing countries, such as Brazil and
Colombia, for example, people start drinking coffee with milk early in childhood. In Brazil, a project
to stimulate the children’s coffee and milk consumption was launched in 2007 and continues to exist
to date [460]. Such a small quantity (20 mL coffee per 200 mL cup) is supposed to be safe and has
helped increase attention and learning capacity of children and teenagers in these countries. While
limiting caffeine to teenagers to a great extent would be ideal, due to cultural habits and to the
increasing demands placed on this age group regarding school, sports, and even work in some
Beverages 2019, 5, 37 28 of 51
places, caffeine consumption is becoming more common among them. According to the American
Academy of Pediatrics [461], for all the above reasons and because of possible unknown medical
conditions, developing teens should consume no more than 100 mg of caffeine daily.
Table 5. Caffeine intake safety limit for different age groups recommended by studies and regulatory
agencies around the world.
Agency or Study
Safe Limit of Caffeine Consumption
(mg/day or mg/kg Body Weight
bw/day)
Equivalent to
(Approximately)
Adults
USA: Nawrot et al. [232]
1cup of 100 mL espresso coffee + 2 cups of 100 mL
manual dripped coffee or
6 cups of 100 mL manual dripped coffee or
3 cups of 100 mL French press coffee + 1 cup 100 mL
cold brewing coffee
Canada: Health Canada [462]
South Korea: Korean Food and
Drug Administration [463]
400 mg/day
Belgium: Belgium Superior
Health Council [325]
Europe: European Food Safety
Authority [126]
Europe: European Food Safety
Authority [126]
Single doses of up to 200 mg
3 cups of 100 mL manual dripped coffee or
1 cup of 100 mL soluble coffee + 1 cup 100 mL cold
brewing coffee or
2 cups of 100 mL aero press coffee + 1 cup of 100 mL
infusion coffee bag
Reproductive-aged women
International Life Science
Institute (ILSI) [464]
Less than 5 to 6 mg/kg bw/day
(amounts estimated for a 70 kg bw woman)
1cup of 100mL espresso coffee + 1 cup of 100 mL
French press coffee or
2 cups of 100 mL mocha coffee
Pregnant and lactating woman
USA: Nawrot et al. [232]
1 cup of 100 mL espresso coffee + 1 cup of 100 mL
dripped coffee or
2 cups of 100 mL mocha coffee or
2 cups of 100 mL electric dripper coffee + 1 cup 100 mL
ready to drink coffee beverage
New Zealand: New Zealand
Ministry of Health [452]
South Korea: Korean Food and
Drug Administration [463]
300 mg/day
Belgium: Belgium Superior
Health Council [325]
United Kingdom: UK Food
Standard Agency [465]
Europe: European Food Safety
Authority [126]
200 mg/day
3 cups of 100 mL manual dripped coffee or
1 cup of 100 mL soluble coffee + 1 cup 100mL cold
brewing coffee or
2 cups of 100 mL aero press coffee + 1 cup of 100mL
infusion coffee bag
Children
USA: Nawrot et al. [232]
Canada: Health Canada [462]
(amounts estimated for a child a 5-8-year-old, with 22
kg bw)
1 cup of 100 mL manual dripped coffee or
1 cup of 100 mL ready to drink coffee beverage
South Korea: Korean Food and
Drug Administration [463]
< 2.5 mg/kg bw/day
Belgium: Belgium Superior
Health Council [325]
UK: Knight et al. [466]
45 mg/day (up to 4 years)
1 cup of 50 mL cold brewing coffee or
1 cup of 75 mL of manual dripped coffee
New Zealand: New Zealand
Ministry of Health [452]
95 mg/day (aged 5-12 years)
1 cup of 100mL manual dripped coffee + 1 cup of 50mL
ready to drink coffee beverage
Europe: European Food Safety
Authority [126]
< 3.0 mg/kg bw/day
(amounts estimated for a 5-8-year-old child, with 22 kg
bw)
1 cup of 100 mL manual dripped coffee or
1 cup of 100 mL ready to drink coffee beverage
10. Concluding Remarks
Beverages 2019, 5, 37 29 of 51
In the present review, reports on the contents of caffeine in coffee seeds, commercial ground
roasted coffees, instant coffees, and brews were summarized with focus on the variability of caffeine
content in brews due to cultural habits, which are reflected in the blend composition, extraction
methods, and proportion of powder/water used for brews preparation. Lower amounts of caffeine in
coffee can be obtained by decaffeination processes and low-caffeine cultivars.
Caffeine intake varies significantly considering different types of beverages commonly
consumed in the various cultures and population groups around the world. Coffee typically
contains more caffeine than most other beverages and is widely and frequently consumed. Thus, it
contributes significantly to overall caffeine consumption in populations, particularly in adults and
the elderly. Considering the widespread caffeine consumption around the world, some assessments
have been made in order to establish the maximum safe consumption limit for different population
groups. In this case, the inclusion of caffeine on product labels may prevent its unsafe consumption.
A number of studies have demonstrated that caffeine is rapidly absorbed and extensively
metabolized mainly in the liver by CYP1A2, which is polymorphically distributed in human
populations, causing a considerable difference in clearance time and sensitivity of caffeine’s acute
effect in the body and this is likely to be an issue that warrants further investigation, regarding
health outcomes. Regarding the complexation between caffeine and chlorogenic acids, the
bioacessibility and health effects of these complexes are unknown. Considering that plain caffeine
and caffeine from coffee were reported to exert similar stimulating effects [127], it is more probable
that caffeine is not unbound during digestion and that these complexes are not absorbed. If
absorbed, it is probable that they do not bind adenosine receptors. If not absorbed, it is possible that
such complexes could exert an antioxidative effect in the digestive system, as it occurs when
chlorogenic acids are bound to melanoidins. This subject deserves a thorough investigation.
Caffeine has been the subject of extensive research for its long history of use and elevated
consumption worldwide both in natural foods and in medicines. The combined physiological and
psychological impacts of caffeine intake depend mainly on the individual genotype and on the
pattern and the degree of exposure to the substance. It must also be noted that most mechanistic
explanations on caffeine’s effects have been derived from acute administration to fasting subjects
submitted to a period of caffeine abstinence in order to ensure low plasma caffeine concentrations. It
is thus difficult to extrapolate the results to the usual pattern of caffeine consumption, given that
most people consume it at different intervals throughout the day and over periods of years.
Controversies regarding caffeines benefits and risks still exist, but reliable evidence is becoming
available supporting its health promoting potential when moderate amounts are consumed.
Funding: This research received no external funding other than the scholarships cited bellow.
Acknowledgements: The authors would like to acknowledge the scholarships provided by the National
Council for Scientific and Technological Development (CNPq, Brazil reg.# 309091/2016-0) and the Rio de Janeiro
State Research Support Foundation (FAPERJ: E-02/2017# 234092).
Conflicts of Interest: The authors declare no conflicts of interest.
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